[Patch] Windows relation extension failure at 2GB and 4GB

From d3f7543a35b3b72a7069188302cbfc7e4de9120b Mon Sep 17 00:00:00 2001
From: Bryan Green <dbryan.green@gmail.com>
Date: Thu, 6 Nov 2025 10:56:02 -0600
Subject: [PATCH] Fix Windows file I/O to support files larger than 2GB

PostgreSQL's Windows port has been unable to handle files larger than 2GB
due to pervasive use of off_t for file offsets, which is only 32-bit on
Windows. This causes signed integer overflow at exactly 2^31 bytes.

The codebase already defines pgoff_t as __int64 (64-bit) on Windows for
this purpose, and some function declarations in headers use it, but many
implementations still used off_t.

This issue is unlikely to affect most users since the default RELSEG_SIZE
is 1GB, keeping individual segment files small. However, anyone building
with --with-segsize larger than 2 would hit this bug. Tested with
--with-segsize=8 and verified that files can now grow beyond 4GB.

This version also addresses three additional code paths in WAL handling
that used casts to off_t when calling pg_pread() or pg_pwrite():
- xlogrecovery.c: pg_pread() called with cast to off_t
- xlogreader.c: pg_pread() with cast to off_t
- walreceiver.c: pg_pwrite() with cast to off_t

While these are not critical (WAL segments have a max size of 1GB), the
casts are now corrected to pgoff_t for consistency and to avoid any
potential future issues.

Note: off_t is still used in other parts of the codebase (e.g. buffile.c)
which may have similar issues on Windows, but those are outside the
critical path for relation file extension and can be addressed separately.

On Unix-like systems, pgoff_t is defined as off_t, so this change only
affects Windows behavior.
---
 src/backend/access/transam/xlogreader.c   |  2 +-
 src/backend/access/transam/xlogrecovery.c |  2 +-
 src/backend/replication/walreceiver.c     |  2 +-
 src/backend/storage/file/fd.c             | 38 ++++++++---------
 src/backend/storage/smgr/md.c             | 50 +++++++++++------------
 src/common/file_utils.c                   |  4 +-
 src/include/common/file_utils.h           |  4 +-
 src/include/port/pg_iovec.h               |  4 +-
 src/include/port/win32_port.h             |  4 +-
 src/include/storage/fd.h                  | 26 ++++++------
 src/port/win32pread.c                     | 10 ++---
 src/port/win32pwrite.c                    | 10 ++---
 12 files changed, 78 insertions(+), 78 deletions(-)

diff --git a/src/backend/access/transam/xlogreader.c b/src/backend/access/transam/xlogreader.c
index dcc8d4f9c1..8ea837003f 100644
--- a/src/backend/access/transam/xlogreader.c
+++ b/src/backend/access/transam/xlogreader.c
@@ -1574,7 +1574,7 @@ WALRead(XLogReaderState *state,
 
 		/* Reset errno first; eases reporting non-errno-affecting errors */
 		errno = 0;
-		readbytes = pg_pread(state->seg.ws_file, p, segbytes, (off_t) startoff);
+		readbytes = pg_pread(state->seg.ws_file, p, segbytes, (pgoff_t) startoff);
 
 #ifndef FRONTEND
 		pgstat_report_wait_end();
diff --git a/src/backend/access/transam/xlogrecovery.c b/src/backend/access/transam/xlogrecovery.c
index 550de6e4a5..c723d03d96 100644
--- a/src/backend/access/transam/xlogrecovery.c
+++ b/src/backend/access/transam/xlogrecovery.c
@@ -3429,7 +3429,7 @@ retry:
 	io_start = pgstat_prepare_io_time(track_wal_io_timing);
 
 	pgstat_report_wait_start(WAIT_EVENT_WAL_READ);
-	r = pg_pread(readFile, readBuf, XLOG_BLCKSZ, (off_t) readOff);
+	r = pg_pread(readFile, readBuf, XLOG_BLCKSZ, (pgoff_t) readOff);
 	if (r != XLOG_BLCKSZ)
 	{
 		char		fname[MAXFNAMELEN];
diff --git a/src/backend/replication/walreceiver.c b/src/backend/replication/walreceiver.c
index 7361ffc9dc..ec243db3a4 100644
--- a/src/backend/replication/walreceiver.c
+++ b/src/backend/replication/walreceiver.c
@@ -928,7 +928,7 @@ XLogWalRcvWrite(char *buf, Size nbytes, XLogRecPtr recptr, TimeLineID tli)
 		start = pgstat_prepare_io_time(track_wal_io_timing);
 
 		pgstat_report_wait_start(WAIT_EVENT_WAL_WRITE);
-		byteswritten = pg_pwrite(recvFile, buf, segbytes, (off_t) startoff);
+		byteswritten = pg_pwrite(recvFile, buf, segbytes, (pgoff_t) startoff);
 		pgstat_report_wait_end();
 
 		pgstat_count_io_op_time(IOOBJECT_WAL, IOCONTEXT_NORMAL,
diff --git a/src/backend/storage/file/fd.c b/src/backend/storage/file/fd.c
index a4ec7959f3..b25e74831e 100644
--- a/src/backend/storage/file/fd.c
+++ b/src/backend/storage/file/fd.c
@@ -201,7 +201,7 @@ typedef struct vfd
 	File		nextFree;		/* link to next free VFD, if in freelist */
 	File		lruMoreRecently;	/* doubly linked recency-of-use list */
 	File		lruLessRecently;
-	off_t		fileSize;		/* current size of file (0 if not temporary) */
+	pgoff_t		fileSize;		/* current size of file (0 if not temporary) */
 	char	   *fileName;		/* name of file, or NULL for unused VFD */
 	/* NB: fileName is malloc'd, and must be free'd when closing the VFD */
 	int			fileFlags;		/* open(2) flags for (re)opening the file */
@@ -519,7 +519,7 @@ pg_file_exists(const char *name)
  * offset of 0 with nbytes 0 means that the entire file should be flushed
  */
 void
-pg_flush_data(int fd, off_t offset, off_t nbytes)
+pg_flush_data(int fd, pgoff_t offset, pgoff_t nbytes)
 {
 	/*
 	 * Right now file flushing is primarily used to avoid making later
@@ -635,7 +635,7 @@ retry:
 		 * may simply not be enough address space.  If so, silently fall
 		 * through to the next implementation.
 		 */
-		if (nbytes <= (off_t) SSIZE_MAX)
+		if (nbytes <= (pgoff_t) SSIZE_MAX)
 			p = mmap(NULL, nbytes, PROT_READ, MAP_SHARED, fd, offset);
 		else
 			p = MAP_FAILED;
@@ -697,7 +697,7 @@ retry:
  * Truncate an open file to a given length.
  */
 static int
-pg_ftruncate(int fd, off_t length)
+pg_ftruncate(int fd, pgoff_t length)
 {
 	int			ret;
 
@@ -714,7 +714,7 @@ retry:
  * Truncate a file to a given length by name.
  */
 int
-pg_truncate(const char *path, off_t length)
+pg_truncate(const char *path, pgoff_t length)
 {
 	int			ret;
 #ifdef WIN32
@@ -1526,7 +1526,7 @@ FileAccess(File file)
  * Called whenever a temporary file is deleted to report its size.
  */
 static void
-ReportTemporaryFileUsage(const char *path, off_t size)
+ReportTemporaryFileUsage(const char *path, pgoff_t size)
 {
 	pgstat_report_tempfile(size);
 
@@ -2077,7 +2077,7 @@ FileClose(File file)
  * this.
  */
 int
-FilePrefetch(File file, off_t offset, off_t amount, uint32 wait_event_info)
+FilePrefetch(File file, pgoff_t offset, pgoff_t amount, uint32 wait_event_info)
 {
 	Assert(FileIsValid(file));
 
@@ -2108,7 +2108,7 @@ retry:
 	{
 		struct radvisory
 		{
-			off_t		ra_offset;	/* offset into the file */
+			pgoff_t		ra_offset;	/* offset into the file */
 			int			ra_count;	/* size of the read     */
 		}			ra;
 		int			returnCode;
@@ -2133,7 +2133,7 @@ retry:
 }
 
 void
-FileWriteback(File file, off_t offset, off_t nbytes, uint32 wait_event_info)
+FileWriteback(File file, pgoff_t offset, pgoff_t nbytes, uint32 wait_event_info)
 {
 	int			returnCode;
 
@@ -2159,7 +2159,7 @@ FileWriteback(File file, off_t offset, off_t nbytes, uint32 wait_event_info)
 }
 
 ssize_t
-FileReadV(File file, const struct iovec *iov, int iovcnt, off_t offset,
+FileReadV(File file, const struct iovec *iov, int iovcnt, pgoff_t offset,
 		  uint32 wait_event_info)
 {
 	ssize_t		returnCode;
@@ -2216,7 +2216,7 @@ retry:
 
 int
 FileStartReadV(PgAioHandle *ioh, File file,
-			   int iovcnt, off_t offset,
+			   int iovcnt, pgoff_t offset,
 			   uint32 wait_event_info)
 {
 	int			returnCode;
@@ -2241,7 +2241,7 @@ FileStartReadV(PgAioHandle *ioh, File file,
 }
 
 ssize_t
-FileWriteV(File file, const struct iovec *iov, int iovcnt, off_t offset,
+FileWriteV(File file, const struct iovec *iov, int iovcnt, pgoff_t offset,
 		   uint32 wait_event_info)
 {
 	ssize_t		returnCode;
@@ -2270,7 +2270,7 @@ FileWriteV(File file, const struct iovec *iov, int iovcnt, off_t offset,
 	 */
 	if (temp_file_limit >= 0 && (vfdP->fdstate & FD_TEMP_FILE_LIMIT))
 	{
-		off_t		past_write = offset;
+		pgoff_t		past_write = offset;
 
 		for (int i = 0; i < iovcnt; ++i)
 			past_write += iov[i].iov_len;
@@ -2309,7 +2309,7 @@ retry:
 		 */
 		if (vfdP->fdstate & FD_TEMP_FILE_LIMIT)
 		{
-			off_t		past_write = offset + returnCode;
+			pgoff_t		past_write = offset + returnCode;
 
 			if (past_write > vfdP->fileSize)
 			{
@@ -2373,7 +2373,7 @@ FileSync(File file, uint32 wait_event_info)
  * appropriate error.
  */
 int
-FileZero(File file, off_t offset, off_t amount, uint32 wait_event_info)
+FileZero(File file, pgoff_t offset, pgoff_t amount, uint32 wait_event_info)
 {
 	int			returnCode;
 	ssize_t		written;
@@ -2418,7 +2418,7 @@ FileZero(File file, off_t offset, off_t amount, uint32 wait_event_info)
  * appropriate error.
  */
 int
-FileFallocate(File file, off_t offset, off_t amount, uint32 wait_event_info)
+FileFallocate(File file, pgoff_t offset, pgoff_t amount, uint32 wait_event_info)
 {
 #ifdef HAVE_POSIX_FALLOCATE
 	int			returnCode;
@@ -2457,7 +2457,7 @@ retry:
 	return FileZero(file, offset, amount, wait_event_info);
 }
 
-off_t
+pgoff_t
 FileSize(File file)
 {
 	Assert(FileIsValid(file));
@@ -2468,14 +2468,14 @@ FileSize(File file)
 	if (FileIsNotOpen(file))
 	{
 		if (FileAccess(file) < 0)
-			return (off_t) -1;
+			return (pgoff_t) -1;
 	}
 
 	return lseek(VfdCache[file].fd, 0, SEEK_END);
 }
 
 int
-FileTruncate(File file, off_t offset, uint32 wait_event_info)
+FileTruncate(File file, pgoff_t offset, uint32 wait_event_info)
 {
 	int			returnCode;
 
diff --git a/src/backend/storage/smgr/md.c b/src/backend/storage/smgr/md.c
index 235ba7e191..e3f335a834 100644
--- a/src/backend/storage/smgr/md.c
+++ b/src/backend/storage/smgr/md.c
@@ -487,7 +487,7 @@ void
 mdextend(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 		 const void *buffer, bool skipFsync)
 {
-	off_t		seekpos;
+	pgoff_t		seekpos;
 	int			nbytes;
 	MdfdVec    *v;
 
@@ -515,9 +515,9 @@ mdextend(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 
 	v = _mdfd_getseg(reln, forknum, blocknum, skipFsync, EXTENSION_CREATE);
 
-	seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
+	seekpos = (pgoff_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
 
-	Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);
+	Assert(seekpos < (pgoff_t) BLCKSZ * RELSEG_SIZE);
 
 	if ((nbytes = FileWrite(v->mdfd_vfd, buffer, BLCKSZ, seekpos, WAIT_EVENT_DATA_FILE_EXTEND)) != BLCKSZ)
 	{
@@ -578,7 +578,7 @@ mdzeroextend(SMgrRelation reln, ForkNumber forknum,
 	while (remblocks > 0)
 	{
 		BlockNumber segstartblock = curblocknum % ((BlockNumber) RELSEG_SIZE);
-		off_t		seekpos = (off_t) BLCKSZ * segstartblock;
+		pgoff_t		seekpos = (pgoff_t) BLCKSZ * segstartblock;
 		int			numblocks;
 
 		if (segstartblock + remblocks > RELSEG_SIZE)
@@ -607,7 +607,7 @@ mdzeroextend(SMgrRelation reln, ForkNumber forknum,
 			int			ret;
 
 			ret = FileFallocate(v->mdfd_vfd,
-								seekpos, (off_t) BLCKSZ * numblocks,
+								seekpos, (pgoff_t) BLCKSZ * numblocks,
 								WAIT_EVENT_DATA_FILE_EXTEND);
 			if (ret != 0)
 			{
@@ -630,7 +630,7 @@ mdzeroextend(SMgrRelation reln, ForkNumber forknum,
 			 * whole length of the extension.
 			 */
 			ret = FileZero(v->mdfd_vfd,
-						   seekpos, (off_t) BLCKSZ * numblocks,
+						   seekpos, (pgoff_t) BLCKSZ * numblocks,
 						   WAIT_EVENT_DATA_FILE_EXTEND);
 			if (ret < 0)
 				ereport(ERROR,
@@ -745,7 +745,7 @@ mdprefetch(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 
 	while (nblocks > 0)
 	{
-		off_t		seekpos;
+		pgoff_t		seekpos;
 		MdfdVec    *v;
 		int			nblocks_this_segment;
 
@@ -754,9 +754,9 @@ mdprefetch(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 		if (v == NULL)
 			return false;
 
-		seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
+		seekpos = (pgoff_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
 
-		Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);
+		Assert(seekpos < (pgoff_t) BLCKSZ * RELSEG_SIZE);
 
 		nblocks_this_segment =
 			Min(nblocks,
@@ -851,7 +851,7 @@ mdreadv(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 	{
 		struct iovec iov[PG_IOV_MAX];
 		int			iovcnt;
-		off_t		seekpos;
+		pgoff_t		seekpos;
 		int			nbytes;
 		MdfdVec    *v;
 		BlockNumber nblocks_this_segment;
@@ -861,9 +861,9 @@ mdreadv(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 		v = _mdfd_getseg(reln, forknum, blocknum, false,
 						 EXTENSION_FAIL | EXTENSION_CREATE_RECOVERY);
 
-		seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
+		seekpos = (pgoff_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
 
-		Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);
+		Assert(seekpos < (pgoff_t) BLCKSZ * RELSEG_SIZE);
 
 		nblocks_this_segment =
 			Min(nblocks,
@@ -986,7 +986,7 @@ mdstartreadv(PgAioHandle *ioh,
 			 SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 			 void **buffers, BlockNumber nblocks)
 {
-	off_t		seekpos;
+	pgoff_t		seekpos;
 	MdfdVec    *v;
 	BlockNumber nblocks_this_segment;
 	struct iovec *iov;
@@ -996,9 +996,9 @@ mdstartreadv(PgAioHandle *ioh,
 	v = _mdfd_getseg(reln, forknum, blocknum, false,
 					 EXTENSION_FAIL | EXTENSION_CREATE_RECOVERY);
 
-	seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
+	seekpos = (pgoff_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
 
-	Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);
+	Assert(seekpos < (pgoff_t) BLCKSZ * RELSEG_SIZE);
 
 	nblocks_this_segment =
 		Min(nblocks,
@@ -1068,7 +1068,7 @@ mdwritev(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 	{
 		struct iovec iov[PG_IOV_MAX];
 		int			iovcnt;
-		off_t		seekpos;
+		pgoff_t		seekpos;
 		int			nbytes;
 		MdfdVec    *v;
 		BlockNumber nblocks_this_segment;
@@ -1078,9 +1078,9 @@ mdwritev(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 		v = _mdfd_getseg(reln, forknum, blocknum, skipFsync,
 						 EXTENSION_FAIL | EXTENSION_CREATE_RECOVERY);
 
-		seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
+		seekpos = (pgoff_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
 
-		Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);
+		Assert(seekpos < (pgoff_t) BLCKSZ * RELSEG_SIZE);
 
 		nblocks_this_segment =
 			Min(nblocks,
@@ -1173,7 +1173,7 @@ mdwriteback(SMgrRelation reln, ForkNumber forknum,
 	while (nblocks > 0)
 	{
 		BlockNumber nflush = nblocks;
-		off_t		seekpos;
+		pgoff_t		seekpos;
 		MdfdVec    *v;
 		int			segnum_start,
 					segnum_end;
@@ -1202,9 +1202,9 @@ mdwriteback(SMgrRelation reln, ForkNumber forknum,
 		Assert(nflush >= 1);
 		Assert(nflush <= nblocks);
 
-		seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
+		seekpos = (pgoff_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
 
-		FileWriteback(v->mdfd_vfd, seekpos, (off_t) BLCKSZ * nflush, WAIT_EVENT_DATA_FILE_FLUSH);
+		FileWriteback(v->mdfd_vfd, seekpos, (pgoff_t) BLCKSZ * nflush, WAIT_EVENT_DATA_FILE_FLUSH);
 
 		nblocks -= nflush;
 		blocknum += nflush;
@@ -1348,7 +1348,7 @@ mdtruncate(SMgrRelation reln, ForkNumber forknum,
 			 */
 			BlockNumber lastsegblocks = nblocks - priorblocks;
 
-			if (FileTruncate(v->mdfd_vfd, (off_t) lastsegblocks * BLCKSZ, WAIT_EVENT_DATA_FILE_TRUNCATE) < 0)
+			if (FileTruncate(v->mdfd_vfd, (pgoff_t) lastsegblocks * BLCKSZ, WAIT_EVENT_DATA_FILE_TRUNCATE) < 0)
 				ereport(ERROR,
 						(errcode_for_file_access(),
 						 errmsg("could not truncate file \"%s\" to %u blocks: %m",
@@ -1484,9 +1484,9 @@ mdfd(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum, uint32 *off)
 	v = _mdfd_getseg(reln, forknum, blocknum, false,
 					 EXTENSION_FAIL);
 
-	*off = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
+	*off = (pgoff_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
 
-	Assert(*off < (off_t) BLCKSZ * RELSEG_SIZE);
+	Assert(*off < (pgoff_t) BLCKSZ * RELSEG_SIZE);
 
 	return FileGetRawDesc(v->mdfd_vfd);
 }
@@ -1868,7 +1868,7 @@ _mdfd_getseg(SMgrRelation reln, ForkNumber forknum, BlockNumber blkno,
 static BlockNumber
 _mdnblocks(SMgrRelation reln, ForkNumber forknum, MdfdVec *seg)
 {
-	off_t		len;
+	pgoff_t		len;
 
 	len = FileSize(seg->mdfd_vfd);
 	if (len < 0)
diff --git a/src/common/file_utils.c b/src/common/file_utils.c
index 7b62687a2a..cdf08ab5cb 100644
--- a/src/common/file_utils.c
+++ b/src/common/file_utils.c
@@ -656,7 +656,7 @@ compute_remaining_iovec(struct iovec *destination,
  * error is returned, it is unspecified how much has been written.
  */
 ssize_t
-pg_pwritev_with_retry(int fd, const struct iovec *iov, int iovcnt, off_t offset)
+pg_pwritev_with_retry(int fd, const struct iovec *iov, int iovcnt, pgoff_t offset)
 {
 	struct iovec iov_copy[PG_IOV_MAX];
 	ssize_t		sum = 0;
@@ -706,7 +706,7 @@ pg_pwritev_with_retry(int fd, const struct iovec *iov, int iovcnt, off_t offset)
  * is returned with errno set.
  */
 ssize_t
-pg_pwrite_zeros(int fd, size_t size, off_t offset)
+pg_pwrite_zeros(int fd, size_t size, pgoff_t offset)
 {
 	static const PGIOAlignedBlock zbuffer = {0};	/* worth BLCKSZ */
 	void	   *zerobuf_addr = unconstify(PGIOAlignedBlock *, &zbuffer)->data;
diff --git a/src/include/common/file_utils.h b/src/include/common/file_utils.h
index 9fd88953e4..4239713803 100644
--- a/src/include/common/file_utils.h
+++ b/src/include/common/file_utils.h
@@ -55,9 +55,9 @@ extern int	compute_remaining_iovec(struct iovec *destination,
 extern ssize_t pg_pwritev_with_retry(int fd,
 									 const struct iovec *iov,
 									 int iovcnt,
-									 off_t offset);
+									 pgoff_t offset);
 
-extern ssize_t pg_pwrite_zeros(int fd, size_t size, off_t offset);
+extern ssize_t pg_pwrite_zeros(int fd, size_t size, pgoff_t offset);
 
 /* Filename components */
 #define PG_TEMP_FILES_DIR "pgsql_tmp"
diff --git a/src/include/port/pg_iovec.h b/src/include/port/pg_iovec.h
index 90be3af449..845ded8c71 100644
--- a/src/include/port/pg_iovec.h
+++ b/src/include/port/pg_iovec.h
@@ -51,7 +51,7 @@ struct iovec
  * this changes the current file position.
  */
 static inline ssize_t
-pg_preadv(int fd, const struct iovec *iov, int iovcnt, off_t offset)
+pg_preadv(int fd, const struct iovec *iov, int iovcnt, pgoff_t offset)
 {
 #if HAVE_DECL_PREADV
 	/*
@@ -90,7 +90,7 @@ pg_preadv(int fd, const struct iovec *iov, int iovcnt, off_t offset)
  * this changes the current file position.
  */
 static inline ssize_t
-pg_pwritev(int fd, const struct iovec *iov, int iovcnt, off_t offset)
+pg_pwritev(int fd, const struct iovec *iov, int iovcnt, pgoff_t offset)
 {
 #if HAVE_DECL_PWRITEV
 	/*
diff --git a/src/include/port/win32_port.h b/src/include/port/win32_port.h
index ff7028bdc8..f54ccef7db 100644
--- a/src/include/port/win32_port.h
+++ b/src/include/port/win32_port.h
@@ -584,9 +584,9 @@ typedef unsigned short mode_t;
 #endif
 
 /* in port/win32pread.c */
-extern ssize_t pg_pread(int fd, void *buf, size_t nbyte, off_t offset);
+extern ssize_t pg_pread(int fd, void *buf, size_t nbyte, pgoff_t offset);
 
 /* in port/win32pwrite.c */
-extern ssize_t pg_pwrite(int fd, const void *buf, size_t nbyte, off_t offset);
+extern ssize_t pg_pwrite(int fd, const void *buf, size_t nbyte, pgoff_t offset);
 
 #endif							/* PG_WIN32_PORT_H */
diff --git a/src/include/storage/fd.h b/src/include/storage/fd.h
index b77d8e5e30..3e821ce8fb 100644
--- a/src/include/storage/fd.h
+++ b/src/include/storage/fd.h
@@ -108,17 +108,17 @@ extern File PathNameOpenFile(const char *fileName, int fileFlags);
 extern File PathNameOpenFilePerm(const char *fileName, int fileFlags, mode_t fileMode);
 extern File OpenTemporaryFile(bool interXact);
 extern void FileClose(File file);
-extern int	FilePrefetch(File file, off_t offset, off_t amount, uint32 wait_event_info);
-extern ssize_t FileReadV(File file, const struct iovec *iov, int iovcnt, off_t offset, uint32 wait_event_info);
-extern ssize_t FileWriteV(File file, const struct iovec *iov, int iovcnt, off_t offset, uint32 wait_event_info);
-extern int	FileStartReadV(struct PgAioHandle *ioh, File file, int iovcnt, off_t offset, uint32 wait_event_info);
+extern int	FilePrefetch(File file, pgoff_t offset, pgoff_t amount, uint32 wait_event_info);
+extern ssize_t FileReadV(File file, const struct iovec *iov, int iovcnt, pgoff_t offset, uint32 wait_event_info);
+extern ssize_t FileWriteV(File file, const struct iovec *iov, int iovcnt, pgoff_t offset, uint32 wait_event_info);
+extern int	FileStartReadV(struct PgAioHandle *ioh, File file, int iovcnt, pgoff_t offset, uint32 wait_event_info);
 extern int	FileSync(File file, uint32 wait_event_info);
-extern int	FileZero(File file, off_t offset, off_t amount, uint32 wait_event_info);
-extern int	FileFallocate(File file, off_t offset, off_t amount, uint32 wait_event_info);
+extern int	FileZero(File file, pgoff_t offset, pgoff_t amount, uint32 wait_event_info);
+extern int	FileFallocate(File file, pgoff_t offset, pgoff_t amount, uint32 wait_event_info);
 
-extern off_t FileSize(File file);
-extern int	FileTruncate(File file, off_t offset, uint32 wait_event_info);
-extern void FileWriteback(File file, off_t offset, off_t nbytes, uint32 wait_event_info);
+extern pgoff_t FileSize(File file);
+extern int	FileTruncate(File file, pgoff_t offset, uint32 wait_event_info);
+extern void FileWriteback(File file, pgoff_t offset, pgoff_t nbytes, uint32 wait_event_info);
 extern char *FilePathName(File file);
 extern int	FileGetRawDesc(File file);
 extern int	FileGetRawFlags(File file);
@@ -186,8 +186,8 @@ extern int	pg_fsync_no_writethrough(int fd);
 extern int	pg_fsync_writethrough(int fd);
 extern int	pg_fdatasync(int fd);
 extern bool pg_file_exists(const char *name);
-extern void pg_flush_data(int fd, off_t offset, off_t nbytes);
-extern int	pg_truncate(const char *path, off_t length);
+extern void pg_flush_data(int fd, pgoff_t offset, pgoff_t nbytes);
+extern int	pg_truncate(const char *path, pgoff_t length);
 extern void fsync_fname(const char *fname, bool isdir);
 extern int	fsync_fname_ext(const char *fname, bool isdir, bool ignore_perm, int elevel);
 extern int	durable_rename(const char *oldfile, const char *newfile, int elevel);
@@ -196,7 +196,7 @@ extern void SyncDataDirectory(void);
 extern int	data_sync_elevel(int elevel);
 
 static inline ssize_t
-FileRead(File file, void *buffer, size_t amount, off_t offset,
+FileRead(File file, void *buffer, size_t amount, pgoff_t offset,
 		 uint32 wait_event_info)
 {
 	struct iovec iov = {
@@ -208,7 +208,7 @@ FileRead(File file, void *buffer, size_t amount, off_t offset,
 }
 
 static inline ssize_t
-FileWrite(File file, const void *buffer, size_t amount, off_t offset,
+FileWrite(File file, const void *buffer, size_t amount, pgoff_t offset,
 		  uint32 wait_event_info)
 {
 	struct iovec iov = {
diff --git a/src/port/win32pread.c b/src/port/win32pread.c
index 32d56c462e..1f00dfd8e6 100644
--- a/src/port/win32pread.c
+++ b/src/port/win32pread.c
@@ -17,7 +17,7 @@
 #include <windows.h>
 
 ssize_t
-pg_pread(int fd, void *buf, size_t size, off_t offset)
+pg_pread(int fd, void *buf, size_t size, pgoff_t offset)
 {
 	OVERLAPPED	overlapped = {0};
 	HANDLE		handle;
@@ -30,16 +30,16 @@ pg_pread(int fd, void *buf, size_t size, off_t offset)
 		return -1;
 	}
 
-	/* Avoid overflowing DWORD. */
+	/* Avoid overflowing DWORD */
 	size = Min(size, 1024 * 1024 * 1024);
 
-	/* Note that this changes the file position, despite not using it. */
-	overlapped.Offset = offset;
+	overlapped.Offset = (DWORD) offset;
+	overlapped.OffsetHigh = (DWORD) (offset >> 32);
+
 	if (!ReadFile(handle, buf, size, &result, &overlapped))
 	{
 		if (GetLastError() == ERROR_HANDLE_EOF)
 			return 0;
-
 		_dosmaperr(GetLastError());
 		return -1;
 	}
diff --git a/src/port/win32pwrite.c b/src/port/win32pwrite.c
index 249aa6c468..d9a0d23c2b 100644
--- a/src/port/win32pwrite.c
+++ b/src/port/win32pwrite.c
@@ -15,9 +15,8 @@
 #include "c.h"
 
 #include <windows.h>
-
 ssize_t
-pg_pwrite(int fd, const void *buf, size_t size, off_t offset)
+pg_pwrite(int fd, const void *buf, size_t size, pgoff_t offset)
 {
 	OVERLAPPED	overlapped = {0};
 	HANDLE		handle;
@@ -30,11 +29,12 @@ pg_pwrite(int fd, const void *buf, size_t size, off_t offset)
 		return -1;
 	}
 
-	/* Avoid overflowing DWORD. */
+	/* Avoid overflowing DWORD */
 	size = Min(size, 1024 * 1024 * 1024);
 
-	/* Note that this changes the file position, despite not using it. */
-	overlapped.Offset = offset;
+	overlapped.Offset = (DWORD) offset;
+	overlapped.OffsetHigh = (DWORD) (offset >> 32);
+
 	if (!WriteFile(handle, buf, size, &result, &overlapped))
 	{
 		_dosmaperr(GetLastError());
-- 
2.46.0.windows.1

From 76e97361f7fa45008ec524f0a83eab5c3da46506 Mon Sep 17 00:00:00 2001
From: Bryan Green <dbryan.green@gmail.com>
Date: Thu, 6 Nov 2025 10:56:02 -0600
Subject: [PATCH v3] Fix Windows file I/O to support files larger than 2GB

PostgreSQL's Windows port has been unable to handle files larger than 2GB
due to pervasive use of off_t for file offsets, which is only 32-bit on
Windows. This causes signed integer overflow at exactly 2^31 bytes.

The codebase already defines pgoff_t as __int64 (64-bit) on Windows for
this purpose, and some function declarations in headers use it, but many
implementations still used off_t.

This issue is unlikely to affect most users since the default RELSEG_SIZE
is 1GB, keeping individual segment files small. However, anyone building
with --with-segsize larger than 2 would hit this bug. Tested with
--with-segsize=8 and verified that files can now grow beyond 4GB.

This version also addresses three additional code paths in WAL handling
that used casts to off_t when calling pg_pread() or pg_pwrite():
- xlogrecovery.c: pg_pread() called with cast to off_t
- xlogreader.c: pg_pread() with cast to off_t
- walreceiver.c: pg_pwrite() with cast to off_t

While these are not critical (WAL segments have a max size of 1GB), the
casts are now corrected to pgoff_t for consistency and to avoid any
potential future issues.

Note: off_t is still used in other parts of the codebase (e.g. buffile.c)
which may have similar issues on Windows, but those are outside the
critical path for relation file extension and can be addressed separately.

On Unix-like systems, pgoff_t is defined as off_t, so this change only
affects Windows behavior.
---
 src/backend/access/transam/xlogreader.c       |   2 +-
 src/backend/access/transam/xlogrecovery.c     |   2 +-
 src/backend/replication/walreceiver.c         |   2 +-
 src/backend/storage/file/fd.c                 |  38 +--
 src/backend/storage/smgr/md.c                 |  50 ++--
 src/common/file_utils.c                       |   4 +-
 src/include/common/file_utils.h               |   4 +-
 src/include/port/pg_iovec.h                   |   4 +-
 src/include/port/win32_port.h                 |   4 +-
 src/include/storage/fd.h                      |  26 +-
 src/port/win32pread.c                         |  10 +-
 src/port/win32pwrite.c                        |  10 +-
 src/test/modules/meson.build                  |   1 +
 src/test/modules/test_large_files/Makefile    |  20 ++
 src/test/modules/test_large_files/README      |  53 ++++
 src/test/modules/test_large_files/meson.build |  29 ++
 .../t/001_windows_large_files.pl              |  65 +++++
 .../test_large_files--1.0.sql                 |  36 +++
 .../test_large_files/test_large_files.c       | 270 ++++++++++++++++++
 .../test_large_files/test_large_files.control |   5 +
 20 files changed, 557 insertions(+), 78 deletions(-)
 create mode 100644 src/test/modules/test_large_files/Makefile
 create mode 100644 src/test/modules/test_large_files/README
 create mode 100644 src/test/modules/test_large_files/meson.build
 create mode 100644 src/test/modules/test_large_files/t/001_windows_large_files.pl
 create mode 100644 src/test/modules/test_large_files/test_large_files--1.0.sql
 create mode 100644 src/test/modules/test_large_files/test_large_files.c
 create mode 100644 src/test/modules/test_large_files/test_large_files.control

diff --git a/src/backend/access/transam/xlogreader.c b/src/backend/access/transam/xlogreader.c
index dcc8d4f9c1..8ea837003f 100644
--- a/src/backend/access/transam/xlogreader.c
+++ b/src/backend/access/transam/xlogreader.c
@@ -1574,7 +1574,7 @@ WALRead(XLogReaderState *state,
 
 		/* Reset errno first; eases reporting non-errno-affecting errors */
 		errno = 0;
-		readbytes = pg_pread(state->seg.ws_file, p, segbytes, (off_t) startoff);
+		readbytes = pg_pread(state->seg.ws_file, p, segbytes, (pgoff_t) startoff);
 
 #ifndef FRONTEND
 		pgstat_report_wait_end();
diff --git a/src/backend/access/transam/xlogrecovery.c b/src/backend/access/transam/xlogrecovery.c
index 550de6e4a5..c723d03d96 100644
--- a/src/backend/access/transam/xlogrecovery.c
+++ b/src/backend/access/transam/xlogrecovery.c
@@ -3429,7 +3429,7 @@ retry:
 	io_start = pgstat_prepare_io_time(track_wal_io_timing);
 
 	pgstat_report_wait_start(WAIT_EVENT_WAL_READ);
-	r = pg_pread(readFile, readBuf, XLOG_BLCKSZ, (off_t) readOff);
+	r = pg_pread(readFile, readBuf, XLOG_BLCKSZ, (pgoff_t) readOff);
 	if (r != XLOG_BLCKSZ)
 	{
 		char		fname[MAXFNAMELEN];
diff --git a/src/backend/replication/walreceiver.c b/src/backend/replication/walreceiver.c
index 7361ffc9dc..ec243db3a4 100644
--- a/src/backend/replication/walreceiver.c
+++ b/src/backend/replication/walreceiver.c
@@ -928,7 +928,7 @@ XLogWalRcvWrite(char *buf, Size nbytes, XLogRecPtr recptr, TimeLineID tli)
 		start = pgstat_prepare_io_time(track_wal_io_timing);
 
 		pgstat_report_wait_start(WAIT_EVENT_WAL_WRITE);
-		byteswritten = pg_pwrite(recvFile, buf, segbytes, (off_t) startoff);
+		byteswritten = pg_pwrite(recvFile, buf, segbytes, (pgoff_t) startoff);
 		pgstat_report_wait_end();
 
 		pgstat_count_io_op_time(IOOBJECT_WAL, IOCONTEXT_NORMAL,
diff --git a/src/backend/storage/file/fd.c b/src/backend/storage/file/fd.c
index a4ec7959f3..b25e74831e 100644
--- a/src/backend/storage/file/fd.c
+++ b/src/backend/storage/file/fd.c
@@ -201,7 +201,7 @@ typedef struct vfd
 	File		nextFree;		/* link to next free VFD, if in freelist */
 	File		lruMoreRecently;	/* doubly linked recency-of-use list */
 	File		lruLessRecently;
-	off_t		fileSize;		/* current size of file (0 if not temporary) */
+	pgoff_t		fileSize;		/* current size of file (0 if not temporary) */
 	char	   *fileName;		/* name of file, or NULL for unused VFD */
 	/* NB: fileName is malloc'd, and must be free'd when closing the VFD */
 	int			fileFlags;		/* open(2) flags for (re)opening the file */
@@ -519,7 +519,7 @@ pg_file_exists(const char *name)
  * offset of 0 with nbytes 0 means that the entire file should be flushed
  */
 void
-pg_flush_data(int fd, off_t offset, off_t nbytes)
+pg_flush_data(int fd, pgoff_t offset, pgoff_t nbytes)
 {
 	/*
 	 * Right now file flushing is primarily used to avoid making later
@@ -635,7 +635,7 @@ retry:
 		 * may simply not be enough address space.  If so, silently fall
 		 * through to the next implementation.
 		 */
-		if (nbytes <= (off_t) SSIZE_MAX)
+		if (nbytes <= (pgoff_t) SSIZE_MAX)
 			p = mmap(NULL, nbytes, PROT_READ, MAP_SHARED, fd, offset);
 		else
 			p = MAP_FAILED;
@@ -697,7 +697,7 @@ retry:
  * Truncate an open file to a given length.
  */
 static int
-pg_ftruncate(int fd, off_t length)
+pg_ftruncate(int fd, pgoff_t length)
 {
 	int			ret;
 
@@ -714,7 +714,7 @@ retry:
  * Truncate a file to a given length by name.
  */
 int
-pg_truncate(const char *path, off_t length)
+pg_truncate(const char *path, pgoff_t length)
 {
 	int			ret;
 #ifdef WIN32
@@ -1526,7 +1526,7 @@ FileAccess(File file)
  * Called whenever a temporary file is deleted to report its size.
  */
 static void
-ReportTemporaryFileUsage(const char *path, off_t size)
+ReportTemporaryFileUsage(const char *path, pgoff_t size)
 {
 	pgstat_report_tempfile(size);
 
@@ -2077,7 +2077,7 @@ FileClose(File file)
  * this.
  */
 int
-FilePrefetch(File file, off_t offset, off_t amount, uint32 wait_event_info)
+FilePrefetch(File file, pgoff_t offset, pgoff_t amount, uint32 wait_event_info)
 {
 	Assert(FileIsValid(file));
 
@@ -2108,7 +2108,7 @@ retry:
 	{
 		struct radvisory
 		{
-			off_t		ra_offset;	/* offset into the file */
+			pgoff_t		ra_offset;	/* offset into the file */
 			int			ra_count;	/* size of the read     */
 		}			ra;
 		int			returnCode;
@@ -2133,7 +2133,7 @@ retry:
 }
 
 void
-FileWriteback(File file, off_t offset, off_t nbytes, uint32 wait_event_info)
+FileWriteback(File file, pgoff_t offset, pgoff_t nbytes, uint32 wait_event_info)
 {
 	int			returnCode;
 
@@ -2159,7 +2159,7 @@ FileWriteback(File file, off_t offset, off_t nbytes, uint32 wait_event_info)
 }
 
 ssize_t
-FileReadV(File file, const struct iovec *iov, int iovcnt, off_t offset,
+FileReadV(File file, const struct iovec *iov, int iovcnt, pgoff_t offset,
 		  uint32 wait_event_info)
 {
 	ssize_t		returnCode;
@@ -2216,7 +2216,7 @@ retry:
 
 int
 FileStartReadV(PgAioHandle *ioh, File file,
-			   int iovcnt, off_t offset,
+			   int iovcnt, pgoff_t offset,
 			   uint32 wait_event_info)
 {
 	int			returnCode;
@@ -2241,7 +2241,7 @@ FileStartReadV(PgAioHandle *ioh, File file,
 }
 
 ssize_t
-FileWriteV(File file, const struct iovec *iov, int iovcnt, off_t offset,
+FileWriteV(File file, const struct iovec *iov, int iovcnt, pgoff_t offset,
 		   uint32 wait_event_info)
 {
 	ssize_t		returnCode;
@@ -2270,7 +2270,7 @@ FileWriteV(File file, const struct iovec *iov, int iovcnt, off_t offset,
 	 */
 	if (temp_file_limit >= 0 && (vfdP->fdstate & FD_TEMP_FILE_LIMIT))
 	{
-		off_t		past_write = offset;
+		pgoff_t		past_write = offset;
 
 		for (int i = 0; i < iovcnt; ++i)
 			past_write += iov[i].iov_len;
@@ -2309,7 +2309,7 @@ retry:
 		 */
 		if (vfdP->fdstate & FD_TEMP_FILE_LIMIT)
 		{
-			off_t		past_write = offset + returnCode;
+			pgoff_t		past_write = offset + returnCode;
 
 			if (past_write > vfdP->fileSize)
 			{
@@ -2373,7 +2373,7 @@ FileSync(File file, uint32 wait_event_info)
  * appropriate error.
  */
 int
-FileZero(File file, off_t offset, off_t amount, uint32 wait_event_info)
+FileZero(File file, pgoff_t offset, pgoff_t amount, uint32 wait_event_info)
 {
 	int			returnCode;
 	ssize_t		written;
@@ -2418,7 +2418,7 @@ FileZero(File file, off_t offset, off_t amount, uint32 wait_event_info)
  * appropriate error.
  */
 int
-FileFallocate(File file, off_t offset, off_t amount, uint32 wait_event_info)
+FileFallocate(File file, pgoff_t offset, pgoff_t amount, uint32 wait_event_info)
 {
 #ifdef HAVE_POSIX_FALLOCATE
 	int			returnCode;
@@ -2457,7 +2457,7 @@ retry:
 	return FileZero(file, offset, amount, wait_event_info);
 }
 
-off_t
+pgoff_t
 FileSize(File file)
 {
 	Assert(FileIsValid(file));
@@ -2468,14 +2468,14 @@ FileSize(File file)
 	if (FileIsNotOpen(file))
 	{
 		if (FileAccess(file) < 0)
-			return (off_t) -1;
+			return (pgoff_t) -1;
 	}
 
 	return lseek(VfdCache[file].fd, 0, SEEK_END);
 }
 
 int
-FileTruncate(File file, off_t offset, uint32 wait_event_info)
+FileTruncate(File file, pgoff_t offset, uint32 wait_event_info)
 {
 	int			returnCode;
 
diff --git a/src/backend/storage/smgr/md.c b/src/backend/storage/smgr/md.c
index 235ba7e191..e3f335a834 100644
--- a/src/backend/storage/smgr/md.c
+++ b/src/backend/storage/smgr/md.c
@@ -487,7 +487,7 @@ void
 mdextend(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 		 const void *buffer, bool skipFsync)
 {
-	off_t		seekpos;
+	pgoff_t		seekpos;
 	int			nbytes;
 	MdfdVec    *v;
 
@@ -515,9 +515,9 @@ mdextend(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 
 	v = _mdfd_getseg(reln, forknum, blocknum, skipFsync, EXTENSION_CREATE);
 
-	seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
+	seekpos = (pgoff_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
 
-	Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);
+	Assert(seekpos < (pgoff_t) BLCKSZ * RELSEG_SIZE);
 
 	if ((nbytes = FileWrite(v->mdfd_vfd, buffer, BLCKSZ, seekpos, WAIT_EVENT_DATA_FILE_EXTEND)) != BLCKSZ)
 	{
@@ -578,7 +578,7 @@ mdzeroextend(SMgrRelation reln, ForkNumber forknum,
 	while (remblocks > 0)
 	{
 		BlockNumber segstartblock = curblocknum % ((BlockNumber) RELSEG_SIZE);
-		off_t		seekpos = (off_t) BLCKSZ * segstartblock;
+		pgoff_t		seekpos = (pgoff_t) BLCKSZ * segstartblock;
 		int			numblocks;
 
 		if (segstartblock + remblocks > RELSEG_SIZE)
@@ -607,7 +607,7 @@ mdzeroextend(SMgrRelation reln, ForkNumber forknum,
 			int			ret;
 
 			ret = FileFallocate(v->mdfd_vfd,
-								seekpos, (off_t) BLCKSZ * numblocks,
+								seekpos, (pgoff_t) BLCKSZ * numblocks,
 								WAIT_EVENT_DATA_FILE_EXTEND);
 			if (ret != 0)
 			{
@@ -630,7 +630,7 @@ mdzeroextend(SMgrRelation reln, ForkNumber forknum,
 			 * whole length of the extension.
 			 */
 			ret = FileZero(v->mdfd_vfd,
-						   seekpos, (off_t) BLCKSZ * numblocks,
+						   seekpos, (pgoff_t) BLCKSZ * numblocks,
 						   WAIT_EVENT_DATA_FILE_EXTEND);
 			if (ret < 0)
 				ereport(ERROR,
@@ -745,7 +745,7 @@ mdprefetch(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 
 	while (nblocks > 0)
 	{
-		off_t		seekpos;
+		pgoff_t		seekpos;
 		MdfdVec    *v;
 		int			nblocks_this_segment;
 
@@ -754,9 +754,9 @@ mdprefetch(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 		if (v == NULL)
 			return false;
 
-		seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
+		seekpos = (pgoff_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
 
-		Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);
+		Assert(seekpos < (pgoff_t) BLCKSZ * RELSEG_SIZE);
 
 		nblocks_this_segment =
 			Min(nblocks,
@@ -851,7 +851,7 @@ mdreadv(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 	{
 		struct iovec iov[PG_IOV_MAX];
 		int			iovcnt;
-		off_t		seekpos;
+		pgoff_t		seekpos;
 		int			nbytes;
 		MdfdVec    *v;
 		BlockNumber nblocks_this_segment;
@@ -861,9 +861,9 @@ mdreadv(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 		v = _mdfd_getseg(reln, forknum, blocknum, false,
 						 EXTENSION_FAIL | EXTENSION_CREATE_RECOVERY);
 
-		seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
+		seekpos = (pgoff_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
 
-		Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);
+		Assert(seekpos < (pgoff_t) BLCKSZ * RELSEG_SIZE);
 
 		nblocks_this_segment =
 			Min(nblocks,
@@ -986,7 +986,7 @@ mdstartreadv(PgAioHandle *ioh,
 			 SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 			 void **buffers, BlockNumber nblocks)
 {
-	off_t		seekpos;
+	pgoff_t		seekpos;
 	MdfdVec    *v;
 	BlockNumber nblocks_this_segment;
 	struct iovec *iov;
@@ -996,9 +996,9 @@ mdstartreadv(PgAioHandle *ioh,
 	v = _mdfd_getseg(reln, forknum, blocknum, false,
 					 EXTENSION_FAIL | EXTENSION_CREATE_RECOVERY);
 
-	seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
+	seekpos = (pgoff_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
 
-	Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);
+	Assert(seekpos < (pgoff_t) BLCKSZ * RELSEG_SIZE);
 
 	nblocks_this_segment =
 		Min(nblocks,
@@ -1068,7 +1068,7 @@ mdwritev(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 	{
 		struct iovec iov[PG_IOV_MAX];
 		int			iovcnt;
-		off_t		seekpos;
+		pgoff_t		seekpos;
 		int			nbytes;
 		MdfdVec    *v;
 		BlockNumber nblocks_this_segment;
@@ -1078,9 +1078,9 @@ mdwritev(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 		v = _mdfd_getseg(reln, forknum, blocknum, skipFsync,
 						 EXTENSION_FAIL | EXTENSION_CREATE_RECOVERY);
 
-		seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
+		seekpos = (pgoff_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
 
-		Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);
+		Assert(seekpos < (pgoff_t) BLCKSZ * RELSEG_SIZE);
 
 		nblocks_this_segment =
 			Min(nblocks,
@@ -1173,7 +1173,7 @@ mdwriteback(SMgrRelation reln, ForkNumber forknum,
 	while (nblocks > 0)
 	{
 		BlockNumber nflush = nblocks;
-		off_t		seekpos;
+		pgoff_t		seekpos;
 		MdfdVec    *v;
 		int			segnum_start,
 					segnum_end;
@@ -1202,9 +1202,9 @@ mdwriteback(SMgrRelation reln, ForkNumber forknum,
 		Assert(nflush >= 1);
 		Assert(nflush <= nblocks);
 
-		seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
+		seekpos = (pgoff_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
 
-		FileWriteback(v->mdfd_vfd, seekpos, (off_t) BLCKSZ * nflush, WAIT_EVENT_DATA_FILE_FLUSH);
+		FileWriteback(v->mdfd_vfd, seekpos, (pgoff_t) BLCKSZ * nflush, WAIT_EVENT_DATA_FILE_FLUSH);
 
 		nblocks -= nflush;
 		blocknum += nflush;
@@ -1348,7 +1348,7 @@ mdtruncate(SMgrRelation reln, ForkNumber forknum,
 			 */
 			BlockNumber lastsegblocks = nblocks - priorblocks;
 
-			if (FileTruncate(v->mdfd_vfd, (off_t) lastsegblocks * BLCKSZ, WAIT_EVENT_DATA_FILE_TRUNCATE) < 0)
+			if (FileTruncate(v->mdfd_vfd, (pgoff_t) lastsegblocks * BLCKSZ, WAIT_EVENT_DATA_FILE_TRUNCATE) < 0)
 				ereport(ERROR,
 						(errcode_for_file_access(),
 						 errmsg("could not truncate file \"%s\" to %u blocks: %m",
@@ -1484,9 +1484,9 @@ mdfd(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum, uint32 *off)
 	v = _mdfd_getseg(reln, forknum, blocknum, false,
 					 EXTENSION_FAIL);
 
-	*off = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
+	*off = (pgoff_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
 
-	Assert(*off < (off_t) BLCKSZ * RELSEG_SIZE);
+	Assert(*off < (pgoff_t) BLCKSZ * RELSEG_SIZE);
 
 	return FileGetRawDesc(v->mdfd_vfd);
 }
@@ -1868,7 +1868,7 @@ _mdfd_getseg(SMgrRelation reln, ForkNumber forknum, BlockNumber blkno,
 static BlockNumber
 _mdnblocks(SMgrRelation reln, ForkNumber forknum, MdfdVec *seg)
 {
-	off_t		len;
+	pgoff_t		len;
 
 	len = FileSize(seg->mdfd_vfd);
 	if (len < 0)
diff --git a/src/common/file_utils.c b/src/common/file_utils.c
index 7b62687a2a..cdf08ab5cb 100644
--- a/src/common/file_utils.c
+++ b/src/common/file_utils.c
@@ -656,7 +656,7 @@ compute_remaining_iovec(struct iovec *destination,
  * error is returned, it is unspecified how much has been written.
  */
 ssize_t
-pg_pwritev_with_retry(int fd, const struct iovec *iov, int iovcnt, off_t offset)
+pg_pwritev_with_retry(int fd, const struct iovec *iov, int iovcnt, pgoff_t offset)
 {
 	struct iovec iov_copy[PG_IOV_MAX];
 	ssize_t		sum = 0;
@@ -706,7 +706,7 @@ pg_pwritev_with_retry(int fd, const struct iovec *iov, int iovcnt, off_t offset)
  * is returned with errno set.
  */
 ssize_t
-pg_pwrite_zeros(int fd, size_t size, off_t offset)
+pg_pwrite_zeros(int fd, size_t size, pgoff_t offset)
 {
 	static const PGIOAlignedBlock zbuffer = {0};	/* worth BLCKSZ */
 	void	   *zerobuf_addr = unconstify(PGIOAlignedBlock *, &zbuffer)->data;
diff --git a/src/include/common/file_utils.h b/src/include/common/file_utils.h
index 9fd88953e4..4239713803 100644
--- a/src/include/common/file_utils.h
+++ b/src/include/common/file_utils.h
@@ -55,9 +55,9 @@ extern int	compute_remaining_iovec(struct iovec *destination,
 extern ssize_t pg_pwritev_with_retry(int fd,
 									 const struct iovec *iov,
 									 int iovcnt,
-									 off_t offset);
+									 pgoff_t offset);
 
-extern ssize_t pg_pwrite_zeros(int fd, size_t size, off_t offset);
+extern ssize_t pg_pwrite_zeros(int fd, size_t size, pgoff_t offset);
 
 /* Filename components */
 #define PG_TEMP_FILES_DIR "pgsql_tmp"
diff --git a/src/include/port/pg_iovec.h b/src/include/port/pg_iovec.h
index 90be3af449..845ded8c71 100644
--- a/src/include/port/pg_iovec.h
+++ b/src/include/port/pg_iovec.h
@@ -51,7 +51,7 @@ struct iovec
  * this changes the current file position.
  */
 static inline ssize_t
-pg_preadv(int fd, const struct iovec *iov, int iovcnt, off_t offset)
+pg_preadv(int fd, const struct iovec *iov, int iovcnt, pgoff_t offset)
 {
 #if HAVE_DECL_PREADV
 	/*
@@ -90,7 +90,7 @@ pg_preadv(int fd, const struct iovec *iov, int iovcnt, off_t offset)
  * this changes the current file position.
  */
 static inline ssize_t
-pg_pwritev(int fd, const struct iovec *iov, int iovcnt, off_t offset)
+pg_pwritev(int fd, const struct iovec *iov, int iovcnt, pgoff_t offset)
 {
 #if HAVE_DECL_PWRITEV
 	/*
diff --git a/src/include/port/win32_port.h b/src/include/port/win32_port.h
index ff7028bdc8..f54ccef7db 100644
--- a/src/include/port/win32_port.h
+++ b/src/include/port/win32_port.h
@@ -584,9 +584,9 @@ typedef unsigned short mode_t;
 #endif
 
 /* in port/win32pread.c */
-extern ssize_t pg_pread(int fd, void *buf, size_t nbyte, off_t offset);
+extern ssize_t pg_pread(int fd, void *buf, size_t nbyte, pgoff_t offset);
 
 /* in port/win32pwrite.c */
-extern ssize_t pg_pwrite(int fd, const void *buf, size_t nbyte, off_t offset);
+extern ssize_t pg_pwrite(int fd, const void *buf, size_t nbyte, pgoff_t offset);
 
 #endif							/* PG_WIN32_PORT_H */
diff --git a/src/include/storage/fd.h b/src/include/storage/fd.h
index b77d8e5e30..3e821ce8fb 100644
--- a/src/include/storage/fd.h
+++ b/src/include/storage/fd.h
@@ -108,17 +108,17 @@ extern File PathNameOpenFile(const char *fileName, int fileFlags);
 extern File PathNameOpenFilePerm(const char *fileName, int fileFlags, mode_t fileMode);
 extern File OpenTemporaryFile(bool interXact);
 extern void FileClose(File file);
-extern int	FilePrefetch(File file, off_t offset, off_t amount, uint32 wait_event_info);
-extern ssize_t FileReadV(File file, const struct iovec *iov, int iovcnt, off_t offset, uint32 wait_event_info);
-extern ssize_t FileWriteV(File file, const struct iovec *iov, int iovcnt, off_t offset, uint32 wait_event_info);
-extern int	FileStartReadV(struct PgAioHandle *ioh, File file, int iovcnt, off_t offset, uint32 wait_event_info);
+extern int	FilePrefetch(File file, pgoff_t offset, pgoff_t amount, uint32 wait_event_info);
+extern ssize_t FileReadV(File file, const struct iovec *iov, int iovcnt, pgoff_t offset, uint32 wait_event_info);
+extern ssize_t FileWriteV(File file, const struct iovec *iov, int iovcnt, pgoff_t offset, uint32 wait_event_info);
+extern int	FileStartReadV(struct PgAioHandle *ioh, File file, int iovcnt, pgoff_t offset, uint32 wait_event_info);
 extern int	FileSync(File file, uint32 wait_event_info);
-extern int	FileZero(File file, off_t offset, off_t amount, uint32 wait_event_info);
-extern int	FileFallocate(File file, off_t offset, off_t amount, uint32 wait_event_info);
+extern int	FileZero(File file, pgoff_t offset, pgoff_t amount, uint32 wait_event_info);
+extern int	FileFallocate(File file, pgoff_t offset, pgoff_t amount, uint32 wait_event_info);
 
-extern off_t FileSize(File file);
-extern int	FileTruncate(File file, off_t offset, uint32 wait_event_info);
-extern void FileWriteback(File file, off_t offset, off_t nbytes, uint32 wait_event_info);
+extern pgoff_t FileSize(File file);
+extern int	FileTruncate(File file, pgoff_t offset, uint32 wait_event_info);
+extern void FileWriteback(File file, pgoff_t offset, pgoff_t nbytes, uint32 wait_event_info);
 extern char *FilePathName(File file);
 extern int	FileGetRawDesc(File file);
 extern int	FileGetRawFlags(File file);
@@ -186,8 +186,8 @@ extern int	pg_fsync_no_writethrough(int fd);
 extern int	pg_fsync_writethrough(int fd);
 extern int	pg_fdatasync(int fd);
 extern bool pg_file_exists(const char *name);
-extern void pg_flush_data(int fd, off_t offset, off_t nbytes);
-extern int	pg_truncate(const char *path, off_t length);
+extern void pg_flush_data(int fd, pgoff_t offset, pgoff_t nbytes);
+extern int	pg_truncate(const char *path, pgoff_t length);
 extern void fsync_fname(const char *fname, bool isdir);
 extern int	fsync_fname_ext(const char *fname, bool isdir, bool ignore_perm, int elevel);
 extern int	durable_rename(const char *oldfile, const char *newfile, int elevel);
@@ -196,7 +196,7 @@ extern void SyncDataDirectory(void);
 extern int	data_sync_elevel(int elevel);
 
 static inline ssize_t
-FileRead(File file, void *buffer, size_t amount, off_t offset,
+FileRead(File file, void *buffer, size_t amount, pgoff_t offset,
 		 uint32 wait_event_info)
 {
 	struct iovec iov = {
@@ -208,7 +208,7 @@ FileRead(File file, void *buffer, size_t amount, off_t offset,
 }
 
 static inline ssize_t
-FileWrite(File file, const void *buffer, size_t amount, off_t offset,
+FileWrite(File file, const void *buffer, size_t amount, pgoff_t offset,
 		  uint32 wait_event_info)
 {
 	struct iovec iov = {
diff --git a/src/port/win32pread.c b/src/port/win32pread.c
index 32d56c462e..1f00dfd8e6 100644
--- a/src/port/win32pread.c
+++ b/src/port/win32pread.c
@@ -17,7 +17,7 @@
 #include <windows.h>
 
 ssize_t
-pg_pread(int fd, void *buf, size_t size, off_t offset)
+pg_pread(int fd, void *buf, size_t size, pgoff_t offset)
 {
 	OVERLAPPED	overlapped = {0};
 	HANDLE		handle;
@@ -30,16 +30,16 @@ pg_pread(int fd, void *buf, size_t size, off_t offset)
 		return -1;
 	}
 
-	/* Avoid overflowing DWORD. */
+	/* Avoid overflowing DWORD */
 	size = Min(size, 1024 * 1024 * 1024);
 
-	/* Note that this changes the file position, despite not using it. */
-	overlapped.Offset = offset;
+	overlapped.Offset = (DWORD) offset;
+	overlapped.OffsetHigh = (DWORD) (offset >> 32);
+
 	if (!ReadFile(handle, buf, size, &result, &overlapped))
 	{
 		if (GetLastError() == ERROR_HANDLE_EOF)
 			return 0;
-
 		_dosmaperr(GetLastError());
 		return -1;
 	}
diff --git a/src/port/win32pwrite.c b/src/port/win32pwrite.c
index 249aa6c468..d9a0d23c2b 100644
--- a/src/port/win32pwrite.c
+++ b/src/port/win32pwrite.c
@@ -15,9 +15,8 @@
 #include "c.h"
 
 #include <windows.h>
-
 ssize_t
-pg_pwrite(int fd, const void *buf, size_t size, off_t offset)
+pg_pwrite(int fd, const void *buf, size_t size, pgoff_t offset)
 {
 	OVERLAPPED	overlapped = {0};
 	HANDLE		handle;
@@ -30,11 +29,12 @@ pg_pwrite(int fd, const void *buf, size_t size, off_t offset)
 		return -1;
 	}
 
-	/* Avoid overflowing DWORD. */
+	/* Avoid overflowing DWORD */
 	size = Min(size, 1024 * 1024 * 1024);
 
-	/* Note that this changes the file position, despite not using it. */
-	overlapped.Offset = offset;
+	overlapped.Offset = (DWORD) offset;
+	overlapped.OffsetHigh = (DWORD) (offset >> 32);
+
 	if (!WriteFile(handle, buf, size, &result, &overlapped))
 	{
 		_dosmaperr(GetLastError());
diff --git a/src/test/modules/meson.build b/src/test/modules/meson.build
index 14fc761c4c..95af220a4d 100644
--- a/src/test/modules/meson.build
+++ b/src/test/modules/meson.build
@@ -28,6 +28,7 @@ subdir('test_ginpostinglist')
 subdir('test_int128')
 subdir('test_integerset')
 subdir('test_json_parser')
+subdir('test_large_files')
 subdir('test_lfind')
 subdir('test_lwlock_tranches')
 subdir('test_misc')
diff --git a/src/test/modules/test_large_files/Makefile b/src/test/modules/test_large_files/Makefile
new file mode 100644
index 0000000000..26bb53a51f
--- /dev/null
+++ b/src/test/modules/test_large_files/Makefile
@@ -0,0 +1,20 @@
+# src/test/modules/test_large_files/Makefile
+
+MODULE_big = test_large_files
+OBJS = test_large_files.o
+
+EXTENSION = test_large_files
+DATA = test_large_files--1.0.sql
+
+REGRESS = test_large_files
+
+ifdef USE_PGXS
+PG_CONFIG = pg_config
+PGXS := $(shell $(PG_CONFIG) --pgxs)
+include $(PGXS)
+else
+subdir = src/test/modules/test_large_files
+top_builddir = ../../../..
+include $(top_builddir)/src/Makefile.global
+include $(top_srcdir)/contrib/contrib-global.mk
+endif
diff --git a/src/test/modules/test_large_files/README b/src/test/modules/test_large_files/README
new file mode 100644
index 0000000000..9df6a2ce84
--- /dev/null
+++ b/src/test/modules/test_large_files/README
@@ -0,0 +1,53 @@
+Test Module for Windows Large File I/O
+
+This test module provides functions to test PostgreSQL's ability to
+handle files larger than 4GB on Windows.
+
+Requirements
+
+- Windows platform
+- PostgreSQL built with segment size greater than 2GB
+- NTFS filesystem (for sparse file support)
+
+Functions
+
+test_create_sparse_file(filename text, size_gb int) RETURNS boolean
+
+Creates a sparse file of the specified size in gigabytes. This allows
+testing large offsets without actually writing gigabytes of data to
+disk.
+
+test_sparse_write_read(filename text, offset_gb float8, test_data text)
+RETURNS boolean
+
+Writes test data at the specified offset (in GB) using PostgreSQL's VFD
+layer (FileWrite), then reads it back using FileRead to verify basic I/O
+functionality.
+
+test_verify_offset_native(filename text, offset_gb float8, expected_data
+text) RETURNS boolean
+
+Critical for validation: Uses native Windows APIs (ReadFile with proper
+OVERLAPPED structure) to verify that data written by PostgreSQL is
+actually at the correct offset. This catches bugs where both write and
+read might use the same incorrect offset calculation (making a broken
+test appear to pass).
+
+Without this verification, a test could pass even with broken offset
+handling if both FileWrite and FileRead make the same mistake.
+
+What the Test Verifies
+
+1. Sparse file creation works on Windows
+2. PostgreSQL's FileWrite can write at offsets > 4GB
+3. PostgreSQL's FileRead can read from offsets > 4GB
+4. Data is actually at the correct offset (verified with native Windows
+   APIs)
+
+The native verification step is critical because without it, a test
+could pass even with broken offset handling. For example, if both
+FileWrite and FileRead truncate offsets to 32 bits, writing at 4.5GB
+would actually write at ~512MB, and reading at 4.5GB would read from
+~512MB - the test would find matching data but at the wrong location.
+The native verification catches this by independently checking the
+actual file offset.
diff --git a/src/test/modules/test_large_files/meson.build b/src/test/modules/test_large_files/meson.build
new file mode 100644
index 0000000000..c755e2cf16
--- /dev/null
+++ b/src/test/modules/test_large_files/meson.build
@@ -0,0 +1,29 @@
+# src/test/modules/test_large_files/meson.build
+
+test_large_files_sources = files(
+  'test_large_files.c',
+)
+
+if host_system == 'windows'
+  test_large_files = shared_module('test_large_files',
+    test_large_files_sources,
+    kwargs: pg_test_mod_args,
+  )
+  test_install_libs += test_large_files
+
+  test_install_data += files(
+    'test_large_files.control',
+    'test_large_files--1.0.sql',
+  )
+
+  tests += {
+    'name': 'test_large_files',
+    'sd': meson.current_source_dir(),
+    'bd': meson.current_build_dir(),
+    'tap': {
+      'tests': [
+        't/001_windows_large_files.pl',
+      ],
+    },
+  }
+endif
diff --git a/src/test/modules/test_large_files/t/001_windows_large_files.pl b/src/test/modules/test_large_files/t/001_windows_large_files.pl
new file mode 100644
index 0000000000..2fb0ef5e36
--- /dev/null
+++ b/src/test/modules/test_large_files/t/001_windows_large_files.pl
@@ -0,0 +1,65 @@
+#!/usr/bin/perl
+# Copyright (c) 2025, PostgreSQL Global Development Group
+
+=pod
+
+=head1 NAME
+
+001_windows_large_files.pl - Test Windows support for files >4GB
+
+=head1 SYNOPSIS
+
+  prove src/test/modules/test_large_files/t/001_windows_large_files.pl
+
+=head1 DESCRIPTION
+
+This test verifies that PostgreSQL on Windows can correctly handle file
+operations at offsets beyond 4GB. This requires PostgreSQL to be
+built with a segment size greater than 2GB.
+
+The test uses sparse files to avoid actually writing gigabytes of data.
+
+=cut
+
+use strict;
+use warnings;
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+use File::Spec;
+use File::Temp;
+
+if ($^O ne 'MSWin32')
+{
+	plan skip_all => 'test is Windows-specific';
+}
+
+plan tests => 4;
+
+my $node = PostgreSQL::Test::Cluster->new('main');
+$node->init;
+$node->start;
+
+$node->safe_psql('postgres', 'CREATE EXTENSION test_large_files;');
+pass("test_large_files extension loaded");
+
+my $tempdir = File::Temp->newdir();
+my $testfile = File::Spec->catfile($tempdir, 'large_file_test.dat');
+
+note "Test file: $testfile";
+
+my $create_result = $node->safe_psql('postgres',
+	"SELECT test_create_sparse_file('$testfile', 5);");
+is($create_result, 't', "Created 5GB sparse file");
+
+my $test_4_5gb = $node->safe_psql('postgres',
+	"SELECT test_sparse_write_read('$testfile', 4.5, 'TEST_DATA_AT_4.5GB');");
+is($test_4_5gb, 't', "Write/read successful at 4.5GB offset");
+
+my $verify_4_5gb = $node->safe_psql('postgres',
+	"SELECT test_verify_offset_native('$testfile', 4.5, 'TEST_DATA_AT_4.5GB');");
+is($verify_4_5gb, 't', "Native verification confirms data at correct 4.5GB offset");
+
+$node->stop;
+
+done_testing();
diff --git a/src/test/modules/test_large_files/test_large_files--1.0.sql b/src/test/modules/test_large_files/test_large_files--1.0.sql
new file mode 100644
index 0000000000..c4db84106c
--- /dev/null
+++ b/src/test/modules/test_large_files/test_large_files--1.0.sql
@@ -0,0 +1,36 @@
+-- src/test/modules/test_large_files/test_large_files--1.0.sql
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "CREATE EXTENSION test_large_files" to load this file. \quit
+
+--
+-- test_create_sparse_file(filename text, size_gb int) returns boolean
+--
+-- Creates a sparse file for testing. Windows only.
+--
+CREATE FUNCTION test_create_sparse_file(filename text, size_gb int)
+RETURNS boolean
+AS 'MODULE_PATHNAME', 'test_create_sparse_file'
+LANGUAGE C STRICT;
+
+--
+-- test_sparse_write_read(filename text, offset_gb numeric, test_data text) returns boolean
+--
+-- Writes data at a large offset and reads it back to verify correctness.
+-- Tests pg_pwrite/pg_pread with offsets beyond 2GB and 4GB. Windows only.
+--
+CREATE FUNCTION test_sparse_write_read(filename text, offset_gb float8, test_data text)
+RETURNS boolean
+AS 'MODULE_PATHNAME', 'test_sparse_write_read'
+LANGUAGE C STRICT;
+
+--
+-- test_verify_offset_native(filename text, offset_gb numeric, expected_data text) returns boolean
+--
+-- Uses native Windows APIs to verify data is at the correct offset.
+-- This ensures PostgreSQL's I/O didn't write to a wrapped/incorrect offset.
+--
+CREATE FUNCTION test_verify_offset_native(filename text, offset_gb float8, expected_data text)
+RETURNS boolean
+AS 'MODULE_PATHNAME', 'test_verify_offset_native'
+LANGUAGE C STRICT;
diff --git a/src/test/modules/test_large_files/test_large_files.c b/src/test/modules/test_large_files/test_large_files.c
new file mode 100644
index 0000000000..531230da4b
--- /dev/null
+++ b/src/test/modules/test_large_files/test_large_files.c
@@ -0,0 +1,270 @@
+/* src/test/modules/test_large_files/test_large_files.c */
+
+#include "postgres.h"
+
+#include "fmgr.h"
+#include "storage/fd.h"
+#include "utils/builtins.h"
+
+#ifdef WIN32
+#include <windows.h>
+#include <winioctl.h>
+#endif
+
+PG_MODULE_MAGIC;
+
+PG_FUNCTION_INFO_V1(test_sparse_write_read);
+PG_FUNCTION_INFO_V1(test_create_sparse_file);
+PG_FUNCTION_INFO_V1(test_verify_offset_native);
+
+/*
+ * test_verify_offset_native(filename text, offset_gb numeric, expected_data text) returns boolean
+ *
+ * Uses native Windows APIs to read data at the specified offset and verify it matches.
+ * This ensures PostgreSQL's I/O functions wrote to the CORRECT offset, not a wrapped one.
+ * Windows only.
+ */
+Datum
+test_verify_offset_native(PG_FUNCTION_ARGS)
+{
+#ifdef WIN32
+	text	   *filename_text = PG_GETARG_TEXT_PP(0);
+	float8		offset_gb = PG_GETARG_FLOAT8(1);
+	text	   *expected_text = PG_GETARG_TEXT_PP(2);
+	char	   *filename;
+	char	   *expected_data;
+	char	   *read_buffer;
+	int			expected_len;
+	int64		offset;
+	HANDLE		hFile;
+	OVERLAPPED	overlapped = {0};
+	DWORD		bytesRead;
+	bool		success = false;
+
+	filename = text_to_cstring(filename_text);
+	expected_data = text_to_cstring(expected_text);
+	expected_len = strlen(expected_data) + 1;
+
+	/* Calculate offset in bytes */
+	offset = (int64) (offset_gb * 1024.0 * 1024.0 * 1024.0);
+
+	/* Open file with native Windows API */
+	hFile = CreateFile(filename,
+					   GENERIC_READ,
+					   FILE_SHARE_READ | FILE_SHARE_WRITE,
+					   NULL,
+					   OPEN_EXISTING,
+					   FILE_ATTRIBUTE_NORMAL,
+					   NULL);
+
+	if (hFile == INVALID_HANDLE_VALUE)
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not open file \"%s\" for verification: %lu",
+						filename, GetLastError())));
+
+	/* Set up OVERLAPPED structure with proper 64-bit offset */
+	overlapped.Offset = (DWORD)(offset & 0xFFFFFFFF);
+	overlapped.OffsetHigh = (DWORD)(offset >> 32);
+
+	/* Allocate read buffer */
+	read_buffer = palloc(expected_len);
+
+	/* Read using native Windows API */
+	if (!ReadFile(hFile, read_buffer, expected_len, &bytesRead, &overlapped))
+	{
+		DWORD error = GetLastError();
+		CloseHandle(hFile);
+		pfree(read_buffer);
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("native ReadFile failed at offset %lld: %lu",
+						offset, error)));
+	}
+
+	if (bytesRead != expected_len)
+	{
+		CloseHandle(hFile);
+		pfree(read_buffer);
+		ereport(ERROR,
+				(errmsg("native ReadFile read %lu bytes, expected %d",
+						bytesRead, expected_len)));
+	}
+
+	/* Verify data matches */
+	success = (memcmp(expected_data, read_buffer, expected_len) == 0);
+
+	pfree(read_buffer);
+	CloseHandle(hFile);
+
+	if (!success)
+		ereport(ERROR,
+				(errmsg("data mismatch at offset %lld: PostgreSQL wrote to wrong location",
+						offset)));
+
+	PG_RETURN_BOOL(success);
+#else
+	ereport(ERROR,
+			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+			 errmsg("this test is only supported on Windows")));
+	PG_RETURN_BOOL(false);
+#endif
+}
+
+/*
+ * test_create_sparse_file(filename text, size_gb int) returns boolean
+ *
+ * Creates a sparse file of the specified size in gigabytes.
+ * Windows only.
+ */
+Datum
+test_create_sparse_file(PG_FUNCTION_ARGS)
+{
+#ifdef WIN32
+	text	   *filename_text = PG_GETARG_TEXT_PP(0);
+	int32		size_gb = PG_GETARG_INT32(1);
+	char	   *filename;
+	HANDLE		hFile;
+	DWORD		bytesReturned;
+	LARGE_INTEGER fileSize;
+	bool		success = false;
+
+	filename = text_to_cstring(filename_text);
+
+	/* Open/create the file */
+	hFile = CreateFile(filename,
+					   GENERIC_WRITE,
+					   0,
+					   NULL,
+					   CREATE_ALWAYS,
+					   FILE_ATTRIBUTE_NORMAL,
+					   NULL);
+
+	if (hFile == INVALID_HANDLE_VALUE)
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not create file \"%s\": %lu",
+						filename, GetLastError())));
+
+	/* Mark as sparse */
+	if (!DeviceIoControl(hFile, FSCTL_SET_SPARSE, NULL, 0, NULL, 0,
+						 &bytesReturned, NULL))
+	{
+		CloseHandle(hFile);
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not set file sparse: %lu", GetLastError())));
+	}
+
+	/* Set file size */
+	fileSize.QuadPart = (int64) size_gb * 1024 * 1024 * 1024;
+	if (!SetFilePointerEx(hFile, fileSize, NULL, FILE_BEGIN))
+	{
+		CloseHandle(hFile);
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not set file pointer: %lu", GetLastError())));
+	}
+
+	if (!SetEndOfFile(hFile))
+	{
+		CloseHandle(hFile);
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not set end of file: %lu", GetLastError())));
+	}
+
+	success = true;
+	CloseHandle(hFile);
+
+	PG_RETURN_BOOL(success);
+#else
+	ereport(ERROR,
+			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+			 errmsg("sparse files are only supported on Windows")));
+	PG_RETURN_BOOL(false);
+#endif
+}
+
+/*
+ * test_sparse_write_read(filename text, offset_gb numeric, test_data text) returns boolean
+ *
+ * Writes test data at the specified offset (in GB) and reads it back to verify.
+ * Tests that pg_pwrite and pg_pread work correctly with large offsets.
+ * Windows only.
+ */
+Datum
+test_sparse_write_read(PG_FUNCTION_ARGS)
+{
+#ifdef WIN32
+	text	   *filename_text = PG_GETARG_TEXT_PP(0);
+	float8		offset_gb = PG_GETARG_FLOAT8(1);
+	text	   *test_data_text = PG_GETARG_TEXT_PP(2);
+	char	   *filename;
+	char	   *test_data;
+	char	   *read_buffer;
+	int			test_data_len;
+	pgoff_t		offset;
+	int			fd;
+	ssize_t		written;
+	ssize_t		nread;
+	bool		success = false;
+
+	filename = text_to_cstring(filename_text);
+	test_data = text_to_cstring(test_data_text);
+	test_data_len = strlen(test_data) + 1;	/* include null terminator */
+
+	/* Calculate offset in bytes */
+	offset = (pgoff_t) (offset_gb * 1024.0 * 1024.0 * 1024.0);
+
+	/* Open the file using PostgreSQL's VFD layer */
+	fd = BasicOpenFile(filename, O_RDWR | PG_BINARY);
+	if (fd < 0)
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not open file \"%s\": %m", filename)));
+
+	/* Write test data at the specified offset using pg_pwrite */
+	written = pg_pwrite(fd, test_data, test_data_len, offset);
+	if (written != test_data_len)
+	{
+		close(fd);
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not write to file at offset %lld: wrote %zd of %d bytes",
+						(long long) offset, written, test_data_len)));
+	}
+
+	/* Allocate buffer for reading */
+	read_buffer = palloc(test_data_len);
+
+	/* Read back the data using pg_pread */
+	nread = pg_pread(fd, read_buffer, test_data_len, offset);
+	if (nread != test_data_len)
+	{
+		close(fd);
+		pfree(read_buffer);
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not read from file at offset %lld: read %zd of %d bytes",
+						(long long) offset, nread, test_data_len)));
+	}
+
+	/* Verify data matches */
+	success = (memcmp(test_data, read_buffer, test_data_len) == 0);
+
+	pfree(read_buffer);
+	close(fd);
+
+	if (!success)
+		ereport(ERROR,
+				(errmsg("data mismatch: read data does not match written data")));
+
+	PG_RETURN_BOOL(success);
+#else
+	ereport(ERROR,
+			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+			 errmsg("this test is only supported on Windows")));
+	PG_RETURN_BOOL(false);
+#endif
+}
diff --git a/src/test/modules/test_large_files/test_large_files.control b/src/test/modules/test_large_files/test_large_files.control
new file mode 100644
index 0000000000..9b0a30974b
--- /dev/null
+++ b/src/test/modules/test_large_files/test_large_files.control
@@ -0,0 +1,5 @@
+# test_large_files extension
+comment = 'Test module for large file I/O on Windows'
+default_version = '1.0'
+module_pathname = '$libdir/test_large_files'
+relocatable = true
-- 
2.49.0

From 88e1d4c56f74c604d3773da4c45c0c592724adc5 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Sat, 8 Nov 2025 18:36:52 +0900
Subject: [PATCH] Add check for large files in meson.build

This check existed in the MSVC scripts that have been removed in v16,
and was missing from meson.

Backpatch-through: 16
---
 meson.build | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/meson.build b/meson.build
index 00c46d400714..c7bc2df2c82f 100644
--- a/meson.build
+++ b/meson.build
@@ -452,6 +452,14 @@ else
   segsize = (get_option('segsize') * 1024 * 1024 * 1024) / blocksize
 endif
 
+# If we don't have largefile support, can't handle segment size >= 2GB.
+if cc.sizeof('off_t', args: test_c_args) < 8
+  segsize_bytes = segsize * blocksize
+  if segsize_bytes >= (2 * 1024 * 1024 * 1024)
+    error('Large file support is not enabled. Segment size cannot be larger than 1GB.')
+  endif
+endif
+
 cdata.set('BLCKSZ', blocksize, description:
 '''Size of a disk block --- this also limits the size of a tuple. You can set
    it bigger if you need bigger tuples (although TOAST should reduce the need
-- 
2.51.0

From 0214bb628176da3c7ebe9258466d41ac5bef3599 Mon Sep 17 00:00:00 2001
From: Bryan Green <dbryan.green@gmail.com>
Date: Thu, 6 Nov 2025 10:56:02 -0600
Subject: [PATCH v4] Add Windows support for files larger than 2GB

PostgreSQL's Windows port has been unable to handle files larger than 2GB
due to pervasive use of off_t for file offsets, which is only 32-bit on
Windows. This causes signed integer overflow at exactly 2^31 bytes.

The codebase already defines pgoff_t as __int64 (64-bit) on Windows for
this purpose, and some function declarations in headers use it, but many
implementations still used off_t.

This issue is unlikely to affect most users since the default RELSEG_SIZE
is 1GB, keeping individual segment files small. However, anyone building
with --with-segsize larger than 2 would hit this bug. Tested with
--with-segsize=8 and verified that files can now grow beyond 4GB.

This version also addresses three additional code paths in WAL handling
that used casts to off_t when calling pg_pread() or pg_pwrite():
- xlogrecovery.c: pg_pread() called with cast to off_t
- xlogreader.c: pg_pread() with cast to off_t
- walreceiver.c: pg_pwrite() with cast to off_t

While these are not critical (WAL segments have a max size of 1GB), the
casts are now corrected to pgoff_t for consistency and to avoid any
potential future issues.

Note: off_t is still used in other parts of the codebase (e.g. buffile.c)
which may have similar issues on Windows, but those are outside the
critical path for relation file extension and can be addressed separately.

On Unix-like systems, pgoff_t is defined as off_t, so this change only
affects Windows behavior.
---
 src/backend/access/transam/xlogreader.c       |   2 +-
 src/backend/access/transam/xlogrecovery.c     |   2 +-
 src/backend/replication/walreceiver.c         |   2 +-
 src/backend/storage/file/fd.c                 |  36 +--
 src/backend/storage/smgr/md.c                 |  50 ++--
 src/common/file_utils.c                       |   4 +-
 src/include/common/file_utils.h               |   4 +-
 src/include/port/pg_iovec.h                   |   4 +-
 src/include/port/win32_port.h                 |   4 +-
 src/include/storage/fd.h                      |  26 +-
 src/port/win32pread.c                         |  10 +-
 src/port/win32pwrite.c                        |  10 +-
 src/test/modules/meson.build                  |   1 +
 src/test/modules/test_large_files/Makefile    |  20 ++
 src/test/modules/test_large_files/README      |  53 ++++
 src/test/modules/test_large_files/meson.build |  29 ++
 .../t/001_windows_large_files.pl              |  65 +++++
 .../test_large_files--1.0.sql                 |  36 +++
 .../test_large_files/test_large_files.c       | 270 ++++++++++++++++++
 .../test_large_files/test_large_files.control |   5 +
 20 files changed, 556 insertions(+), 77 deletions(-)
 create mode 100644 src/test/modules/test_large_files/Makefile
 create mode 100644 src/test/modules/test_large_files/README
 create mode 100644 src/test/modules/test_large_files/meson.build
 create mode 100644 src/test/modules/test_large_files/t/001_windows_large_files.pl
 create mode 100644 src/test/modules/test_large_files/test_large_files--1.0.sql
 create mode 100644 src/test/modules/test_large_files/test_large_files.c
 create mode 100644 src/test/modules/test_large_files/test_large_files.control

diff --git a/src/backend/access/transam/xlogreader.c b/src/backend/access/transam/xlogreader.c
index 755f351143..9cc7488e89 100644
--- a/src/backend/access/transam/xlogreader.c
+++ b/src/backend/access/transam/xlogreader.c
@@ -1574,7 +1574,7 @@ WALRead(XLogReaderState *state,
 
 		/* Reset errno first; eases reporting non-errno-affecting errors */
 		errno = 0;
-		readbytes = pg_pread(state->seg.ws_file, p, segbytes, (off_t) startoff);
+		readbytes = pg_pread(state->seg.ws_file, p, segbytes, (pgoff_t) startoff);
 
 #ifndef FRONTEND
 		pgstat_report_wait_end();
diff --git a/src/backend/access/transam/xlogrecovery.c b/src/backend/access/transam/xlogrecovery.c
index eddc22fc5a..21b8f179ba 100644
--- a/src/backend/access/transam/xlogrecovery.c
+++ b/src/backend/access/transam/xlogrecovery.c
@@ -3429,7 +3429,7 @@ retry:
 	io_start = pgstat_prepare_io_time(track_wal_io_timing);
 
 	pgstat_report_wait_start(WAIT_EVENT_WAL_READ);
-	r = pg_pread(readFile, readBuf, XLOG_BLCKSZ, (off_t) readOff);
+	r = pg_pread(readFile, readBuf, XLOG_BLCKSZ, (pgoff_t) readOff);
 	if (r != XLOG_BLCKSZ)
 	{
 		char		fname[MAXFNAMELEN];
diff --git a/src/backend/replication/walreceiver.c b/src/backend/replication/walreceiver.c
index 2ee8fecee2..4217fc54e2 100644
--- a/src/backend/replication/walreceiver.c
+++ b/src/backend/replication/walreceiver.c
@@ -928,7 +928,7 @@ XLogWalRcvWrite(char *buf, Size nbytes, XLogRecPtr recptr, TimeLineID tli)
 		start = pgstat_prepare_io_time(track_wal_io_timing);
 
 		pgstat_report_wait_start(WAIT_EVENT_WAL_WRITE);
-		byteswritten = pg_pwrite(recvFile, buf, segbytes, (off_t) startoff);
+		byteswritten = pg_pwrite(recvFile, buf, segbytes, (pgoff_t) startoff);
 		pgstat_report_wait_end();
 
 		pgstat_count_io_op_time(IOOBJECT_WAL, IOCONTEXT_NORMAL,
diff --git a/src/backend/storage/file/fd.c b/src/backend/storage/file/fd.c
index a4ec7959f3..e9eaaf9c82 100644
--- a/src/backend/storage/file/fd.c
+++ b/src/backend/storage/file/fd.c
@@ -201,7 +201,7 @@ typedef struct vfd
 	File		nextFree;		/* link to next free VFD, if in freelist */
 	File		lruMoreRecently;	/* doubly linked recency-of-use list */
 	File		lruLessRecently;
-	off_t		fileSize;		/* current size of file (0 if not temporary) */
+	pgoff_t		fileSize;		/* current size of file (0 if not temporary) */
 	char	   *fileName;		/* name of file, or NULL for unused VFD */
 	/* NB: fileName is malloc'd, and must be free'd when closing the VFD */
 	int			fileFlags;		/* open(2) flags for (re)opening the file */
@@ -519,7 +519,7 @@ pg_file_exists(const char *name)
  * offset of 0 with nbytes 0 means that the entire file should be flushed
  */
 void
-pg_flush_data(int fd, off_t offset, off_t nbytes)
+pg_flush_data(int fd, pgoff_t offset, pgoff_t nbytes)
 {
 	/*
 	 * Right now file flushing is primarily used to avoid making later
@@ -635,7 +635,7 @@ retry:
 		 * may simply not be enough address space.  If so, silently fall
 		 * through to the next implementation.
 		 */
-		if (nbytes <= (off_t) SSIZE_MAX)
+		if (nbytes <= (pgoff_t) SSIZE_MAX)
 			p = mmap(NULL, nbytes, PROT_READ, MAP_SHARED, fd, offset);
 		else
 			p = MAP_FAILED;
@@ -697,7 +697,7 @@ retry:
  * Truncate an open file to a given length.
  */
 static int
-pg_ftruncate(int fd, off_t length)
+pg_ftruncate(int fd, pgoff_t length)
 {
 	int			ret;
 
@@ -714,7 +714,7 @@ retry:
  * Truncate a file to a given length by name.
  */
 int
-pg_truncate(const char *path, off_t length)
+pg_truncate(const char *path, pgoff_t length)
 {
 	int			ret;
 #ifdef WIN32
@@ -1526,7 +1526,7 @@ FileAccess(File file)
  * Called whenever a temporary file is deleted to report its size.
  */
 static void
-ReportTemporaryFileUsage(const char *path, off_t size)
+ReportTemporaryFileUsage(const char *path, pgoff_t size)
 {
 	pgstat_report_tempfile(size);
 
@@ -2077,7 +2077,7 @@ FileClose(File file)
  * this.
  */
 int
-FilePrefetch(File file, off_t offset, off_t amount, uint32 wait_event_info)
+FilePrefetch(File file, pgoff_t offset, pgoff_t amount, uint32 wait_event_info)
 {
 	Assert(FileIsValid(file));
 
@@ -2133,7 +2133,7 @@ retry:
 }
 
 void
-FileWriteback(File file, off_t offset, off_t nbytes, uint32 wait_event_info)
+FileWriteback(File file, pgoff_t offset, pgoff_t nbytes, uint32 wait_event_info)
 {
 	int			returnCode;
 
@@ -2159,7 +2159,7 @@ FileWriteback(File file, off_t offset, off_t nbytes, uint32 wait_event_info)
 }
 
 ssize_t
-FileReadV(File file, const struct iovec *iov, int iovcnt, off_t offset,
+FileReadV(File file, const struct iovec *iov, int iovcnt, pgoff_t offset,
 		  uint32 wait_event_info)
 {
 	ssize_t		returnCode;
@@ -2216,7 +2216,7 @@ retry:
 
 int
 FileStartReadV(PgAioHandle *ioh, File file,
-			   int iovcnt, off_t offset,
+			   int iovcnt, pgoff_t offset,
 			   uint32 wait_event_info)
 {
 	int			returnCode;
@@ -2241,7 +2241,7 @@ FileStartReadV(PgAioHandle *ioh, File file,
 }
 
 ssize_t
-FileWriteV(File file, const struct iovec *iov, int iovcnt, off_t offset,
+FileWriteV(File file, const struct iovec *iov, int iovcnt, pgoff_t offset,
 		   uint32 wait_event_info)
 {
 	ssize_t		returnCode;
@@ -2270,7 +2270,7 @@ FileWriteV(File file, const struct iovec *iov, int iovcnt, off_t offset,
 	 */
 	if (temp_file_limit >= 0 && (vfdP->fdstate & FD_TEMP_FILE_LIMIT))
 	{
-		off_t		past_write = offset;
+		pgoff_t		past_write = offset;
 
 		for (int i = 0; i < iovcnt; ++i)
 			past_write += iov[i].iov_len;
@@ -2309,7 +2309,7 @@ retry:
 		 */
 		if (vfdP->fdstate & FD_TEMP_FILE_LIMIT)
 		{
-			off_t		past_write = offset + returnCode;
+			pgoff_t		past_write = offset + returnCode;
 
 			if (past_write > vfdP->fileSize)
 			{
@@ -2373,7 +2373,7 @@ FileSync(File file, uint32 wait_event_info)
  * appropriate error.
  */
 int
-FileZero(File file, off_t offset, off_t amount, uint32 wait_event_info)
+FileZero(File file, pgoff_t offset, pgoff_t amount, uint32 wait_event_info)
 {
 	int			returnCode;
 	ssize_t		written;
@@ -2418,7 +2418,7 @@ FileZero(File file, off_t offset, off_t amount, uint32 wait_event_info)
  * appropriate error.
  */
 int
-FileFallocate(File file, off_t offset, off_t amount, uint32 wait_event_info)
+FileFallocate(File file, pgoff_t offset, pgoff_t amount, uint32 wait_event_info)
 {
 #ifdef HAVE_POSIX_FALLOCATE
 	int			returnCode;
@@ -2457,7 +2457,7 @@ retry:
 	return FileZero(file, offset, amount, wait_event_info);
 }
 
-off_t
+pgoff_t
 FileSize(File file)
 {
 	Assert(FileIsValid(file));
@@ -2468,14 +2468,14 @@ FileSize(File file)
 	if (FileIsNotOpen(file))
 	{
 		if (FileAccess(file) < 0)
-			return (off_t) -1;
+			return (pgoff_t) -1;
 	}
 
 	return lseek(VfdCache[file].fd, 0, SEEK_END);
 }
 
 int
-FileTruncate(File file, off_t offset, uint32 wait_event_info)
+FileTruncate(File file, pgoff_t offset, uint32 wait_event_info)
 {
 	int			returnCode;
 
diff --git a/src/backend/storage/smgr/md.c b/src/backend/storage/smgr/md.c
index 235ba7e191..e3f335a834 100644
--- a/src/backend/storage/smgr/md.c
+++ b/src/backend/storage/smgr/md.c
@@ -487,7 +487,7 @@ void
 mdextend(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 		 const void *buffer, bool skipFsync)
 {
-	off_t		seekpos;
+	pgoff_t		seekpos;
 	int			nbytes;
 	MdfdVec    *v;
 
@@ -515,9 +515,9 @@ mdextend(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 
 	v = _mdfd_getseg(reln, forknum, blocknum, skipFsync, EXTENSION_CREATE);
 
-	seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
+	seekpos = (pgoff_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
 
-	Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);
+	Assert(seekpos < (pgoff_t) BLCKSZ * RELSEG_SIZE);
 
 	if ((nbytes = FileWrite(v->mdfd_vfd, buffer, BLCKSZ, seekpos, WAIT_EVENT_DATA_FILE_EXTEND)) != BLCKSZ)
 	{
@@ -578,7 +578,7 @@ mdzeroextend(SMgrRelation reln, ForkNumber forknum,
 	while (remblocks > 0)
 	{
 		BlockNumber segstartblock = curblocknum % ((BlockNumber) RELSEG_SIZE);
-		off_t		seekpos = (off_t) BLCKSZ * segstartblock;
+		pgoff_t		seekpos = (pgoff_t) BLCKSZ * segstartblock;
 		int			numblocks;
 
 		if (segstartblock + remblocks > RELSEG_SIZE)
@@ -607,7 +607,7 @@ mdzeroextend(SMgrRelation reln, ForkNumber forknum,
 			int			ret;
 
 			ret = FileFallocate(v->mdfd_vfd,
-								seekpos, (off_t) BLCKSZ * numblocks,
+								seekpos, (pgoff_t) BLCKSZ * numblocks,
 								WAIT_EVENT_DATA_FILE_EXTEND);
 			if (ret != 0)
 			{
@@ -630,7 +630,7 @@ mdzeroextend(SMgrRelation reln, ForkNumber forknum,
 			 * whole length of the extension.
 			 */
 			ret = FileZero(v->mdfd_vfd,
-						   seekpos, (off_t) BLCKSZ * numblocks,
+						   seekpos, (pgoff_t) BLCKSZ * numblocks,
 						   WAIT_EVENT_DATA_FILE_EXTEND);
 			if (ret < 0)
 				ereport(ERROR,
@@ -745,7 +745,7 @@ mdprefetch(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 
 	while (nblocks > 0)
 	{
-		off_t		seekpos;
+		pgoff_t		seekpos;
 		MdfdVec    *v;
 		int			nblocks_this_segment;
 
@@ -754,9 +754,9 @@ mdprefetch(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 		if (v == NULL)
 			return false;
 
-		seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
+		seekpos = (pgoff_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
 
-		Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);
+		Assert(seekpos < (pgoff_t) BLCKSZ * RELSEG_SIZE);
 
 		nblocks_this_segment =
 			Min(nblocks,
@@ -851,7 +851,7 @@ mdreadv(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 	{
 		struct iovec iov[PG_IOV_MAX];
 		int			iovcnt;
-		off_t		seekpos;
+		pgoff_t		seekpos;
 		int			nbytes;
 		MdfdVec    *v;
 		BlockNumber nblocks_this_segment;
@@ -861,9 +861,9 @@ mdreadv(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 		v = _mdfd_getseg(reln, forknum, blocknum, false,
 						 EXTENSION_FAIL | EXTENSION_CREATE_RECOVERY);
 
-		seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
+		seekpos = (pgoff_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
 
-		Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);
+		Assert(seekpos < (pgoff_t) BLCKSZ * RELSEG_SIZE);
 
 		nblocks_this_segment =
 			Min(nblocks,
@@ -986,7 +986,7 @@ mdstartreadv(PgAioHandle *ioh,
 			 SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 			 void **buffers, BlockNumber nblocks)
 {
-	off_t		seekpos;
+	pgoff_t		seekpos;
 	MdfdVec    *v;
 	BlockNumber nblocks_this_segment;
 	struct iovec *iov;
@@ -996,9 +996,9 @@ mdstartreadv(PgAioHandle *ioh,
 	v = _mdfd_getseg(reln, forknum, blocknum, false,
 					 EXTENSION_FAIL | EXTENSION_CREATE_RECOVERY);
 
-	seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
+	seekpos = (pgoff_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
 
-	Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);
+	Assert(seekpos < (pgoff_t) BLCKSZ * RELSEG_SIZE);
 
 	nblocks_this_segment =
 		Min(nblocks,
@@ -1068,7 +1068,7 @@ mdwritev(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 	{
 		struct iovec iov[PG_IOV_MAX];
 		int			iovcnt;
-		off_t		seekpos;
+		pgoff_t		seekpos;
 		int			nbytes;
 		MdfdVec    *v;
 		BlockNumber nblocks_this_segment;
@@ -1078,9 +1078,9 @@ mdwritev(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 		v = _mdfd_getseg(reln, forknum, blocknum, skipFsync,
 						 EXTENSION_FAIL | EXTENSION_CREATE_RECOVERY);
 
-		seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
+		seekpos = (pgoff_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
 
-		Assert(seekpos < (off_t) BLCKSZ * RELSEG_SIZE);
+		Assert(seekpos < (pgoff_t) BLCKSZ * RELSEG_SIZE);
 
 		nblocks_this_segment =
 			Min(nblocks,
@@ -1173,7 +1173,7 @@ mdwriteback(SMgrRelation reln, ForkNumber forknum,
 	while (nblocks > 0)
 	{
 		BlockNumber nflush = nblocks;
-		off_t		seekpos;
+		pgoff_t		seekpos;
 		MdfdVec    *v;
 		int			segnum_start,
 					segnum_end;
@@ -1202,9 +1202,9 @@ mdwriteback(SMgrRelation reln, ForkNumber forknum,
 		Assert(nflush >= 1);
 		Assert(nflush <= nblocks);
 
-		seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
+		seekpos = (pgoff_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
 
-		FileWriteback(v->mdfd_vfd, seekpos, (off_t) BLCKSZ * nflush, WAIT_EVENT_DATA_FILE_FLUSH);
+		FileWriteback(v->mdfd_vfd, seekpos, (pgoff_t) BLCKSZ * nflush, WAIT_EVENT_DATA_FILE_FLUSH);
 
 		nblocks -= nflush;
 		blocknum += nflush;
@@ -1348,7 +1348,7 @@ mdtruncate(SMgrRelation reln, ForkNumber forknum,
 			 */
 			BlockNumber lastsegblocks = nblocks - priorblocks;
 
-			if (FileTruncate(v->mdfd_vfd, (off_t) lastsegblocks * BLCKSZ, WAIT_EVENT_DATA_FILE_TRUNCATE) < 0)
+			if (FileTruncate(v->mdfd_vfd, (pgoff_t) lastsegblocks * BLCKSZ, WAIT_EVENT_DATA_FILE_TRUNCATE) < 0)
 				ereport(ERROR,
 						(errcode_for_file_access(),
 						 errmsg("could not truncate file \"%s\" to %u blocks: %m",
@@ -1484,9 +1484,9 @@ mdfd(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum, uint32 *off)
 	v = _mdfd_getseg(reln, forknum, blocknum, false,
 					 EXTENSION_FAIL);
 
-	*off = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
+	*off = (pgoff_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
 
-	Assert(*off < (off_t) BLCKSZ * RELSEG_SIZE);
+	Assert(*off < (pgoff_t) BLCKSZ * RELSEG_SIZE);
 
 	return FileGetRawDesc(v->mdfd_vfd);
 }
@@ -1868,7 +1868,7 @@ _mdfd_getseg(SMgrRelation reln, ForkNumber forknum, BlockNumber blkno,
 static BlockNumber
 _mdnblocks(SMgrRelation reln, ForkNumber forknum, MdfdVec *seg)
 {
-	off_t		len;
+	pgoff_t		len;
 
 	len = FileSize(seg->mdfd_vfd);
 	if (len < 0)
diff --git a/src/common/file_utils.c b/src/common/file_utils.c
index 7b62687a2a..cdf08ab5cb 100644
--- a/src/common/file_utils.c
+++ b/src/common/file_utils.c
@@ -656,7 +656,7 @@ compute_remaining_iovec(struct iovec *destination,
  * error is returned, it is unspecified how much has been written.
  */
 ssize_t
-pg_pwritev_with_retry(int fd, const struct iovec *iov, int iovcnt, off_t offset)
+pg_pwritev_with_retry(int fd, const struct iovec *iov, int iovcnt, pgoff_t offset)
 {
 	struct iovec iov_copy[PG_IOV_MAX];
 	ssize_t		sum = 0;
@@ -706,7 +706,7 @@ pg_pwritev_with_retry(int fd, const struct iovec *iov, int iovcnt, off_t offset)
  * is returned with errno set.
  */
 ssize_t
-pg_pwrite_zeros(int fd, size_t size, off_t offset)
+pg_pwrite_zeros(int fd, size_t size, pgoff_t offset)
 {
 	static const PGIOAlignedBlock zbuffer = {0};	/* worth BLCKSZ */
 	void	   *zerobuf_addr = unconstify(PGIOAlignedBlock *, &zbuffer)->data;
diff --git a/src/include/common/file_utils.h b/src/include/common/file_utils.h
index 9fd88953e4..4239713803 100644
--- a/src/include/common/file_utils.h
+++ b/src/include/common/file_utils.h
@@ -55,9 +55,9 @@ extern int	compute_remaining_iovec(struct iovec *destination,
 extern ssize_t pg_pwritev_with_retry(int fd,
 									 const struct iovec *iov,
 									 int iovcnt,
-									 off_t offset);
+									 pgoff_t offset);
 
-extern ssize_t pg_pwrite_zeros(int fd, size_t size, off_t offset);
+extern ssize_t pg_pwrite_zeros(int fd, size_t size, pgoff_t offset);
 
 /* Filename components */
 #define PG_TEMP_FILES_DIR "pgsql_tmp"
diff --git a/src/include/port/pg_iovec.h b/src/include/port/pg_iovec.h
index 90be3af449..845ded8c71 100644
--- a/src/include/port/pg_iovec.h
+++ b/src/include/port/pg_iovec.h
@@ -51,7 +51,7 @@ struct iovec
  * this changes the current file position.
  */
 static inline ssize_t
-pg_preadv(int fd, const struct iovec *iov, int iovcnt, off_t offset)
+pg_preadv(int fd, const struct iovec *iov, int iovcnt, pgoff_t offset)
 {
 #if HAVE_DECL_PREADV
 	/*
@@ -90,7 +90,7 @@ pg_preadv(int fd, const struct iovec *iov, int iovcnt, off_t offset)
  * this changes the current file position.
  */
 static inline ssize_t
-pg_pwritev(int fd, const struct iovec *iov, int iovcnt, off_t offset)
+pg_pwritev(int fd, const struct iovec *iov, int iovcnt, pgoff_t offset)
 {
 #if HAVE_DECL_PWRITEV
 	/*
diff --git a/src/include/port/win32_port.h b/src/include/port/win32_port.h
index ff7028bdc8..f54ccef7db 100644
--- a/src/include/port/win32_port.h
+++ b/src/include/port/win32_port.h
@@ -584,9 +584,9 @@ typedef unsigned short mode_t;
 #endif
 
 /* in port/win32pread.c */
-extern ssize_t pg_pread(int fd, void *buf, size_t nbyte, off_t offset);
+extern ssize_t pg_pread(int fd, void *buf, size_t nbyte, pgoff_t offset);
 
 /* in port/win32pwrite.c */
-extern ssize_t pg_pwrite(int fd, const void *buf, size_t nbyte, off_t offset);
+extern ssize_t pg_pwrite(int fd, const void *buf, size_t nbyte, pgoff_t offset);
 
 #endif							/* PG_WIN32_PORT_H */
diff --git a/src/include/storage/fd.h b/src/include/storage/fd.h
index b77d8e5e30..3e821ce8fb 100644
--- a/src/include/storage/fd.h
+++ b/src/include/storage/fd.h
@@ -108,17 +108,17 @@ extern File PathNameOpenFile(const char *fileName, int fileFlags);
 extern File PathNameOpenFilePerm(const char *fileName, int fileFlags, mode_t fileMode);
 extern File OpenTemporaryFile(bool interXact);
 extern void FileClose(File file);
-extern int	FilePrefetch(File file, off_t offset, off_t amount, uint32 wait_event_info);
-extern ssize_t FileReadV(File file, const struct iovec *iov, int iovcnt, off_t offset, uint32 wait_event_info);
-extern ssize_t FileWriteV(File file, const struct iovec *iov, int iovcnt, off_t offset, uint32 wait_event_info);
-extern int	FileStartReadV(struct PgAioHandle *ioh, File file, int iovcnt, off_t offset, uint32 wait_event_info);
+extern int	FilePrefetch(File file, pgoff_t offset, pgoff_t amount, uint32 wait_event_info);
+extern ssize_t FileReadV(File file, const struct iovec *iov, int iovcnt, pgoff_t offset, uint32 wait_event_info);
+extern ssize_t FileWriteV(File file, const struct iovec *iov, int iovcnt, pgoff_t offset, uint32 wait_event_info);
+extern int	FileStartReadV(struct PgAioHandle *ioh, File file, int iovcnt, pgoff_t offset, uint32 wait_event_info);
 extern int	FileSync(File file, uint32 wait_event_info);
-extern int	FileZero(File file, off_t offset, off_t amount, uint32 wait_event_info);
-extern int	FileFallocate(File file, off_t offset, off_t amount, uint32 wait_event_info);
+extern int	FileZero(File file, pgoff_t offset, pgoff_t amount, uint32 wait_event_info);
+extern int	FileFallocate(File file, pgoff_t offset, pgoff_t amount, uint32 wait_event_info);
 
-extern off_t FileSize(File file);
-extern int	FileTruncate(File file, off_t offset, uint32 wait_event_info);
-extern void FileWriteback(File file, off_t offset, off_t nbytes, uint32 wait_event_info);
+extern pgoff_t FileSize(File file);
+extern int	FileTruncate(File file, pgoff_t offset, uint32 wait_event_info);
+extern void FileWriteback(File file, pgoff_t offset, pgoff_t nbytes, uint32 wait_event_info);
 extern char *FilePathName(File file);
 extern int	FileGetRawDesc(File file);
 extern int	FileGetRawFlags(File file);
@@ -186,8 +186,8 @@ extern int	pg_fsync_no_writethrough(int fd);
 extern int	pg_fsync_writethrough(int fd);
 extern int	pg_fdatasync(int fd);
 extern bool pg_file_exists(const char *name);
-extern void pg_flush_data(int fd, off_t offset, off_t nbytes);
-extern int	pg_truncate(const char *path, off_t length);
+extern void pg_flush_data(int fd, pgoff_t offset, pgoff_t nbytes);
+extern int	pg_truncate(const char *path, pgoff_t length);
 extern void fsync_fname(const char *fname, bool isdir);
 extern int	fsync_fname_ext(const char *fname, bool isdir, bool ignore_perm, int elevel);
 extern int	durable_rename(const char *oldfile, const char *newfile, int elevel);
@@ -196,7 +196,7 @@ extern void SyncDataDirectory(void);
 extern int	data_sync_elevel(int elevel);
 
 static inline ssize_t
-FileRead(File file, void *buffer, size_t amount, off_t offset,
+FileRead(File file, void *buffer, size_t amount, pgoff_t offset,
 		 uint32 wait_event_info)
 {
 	struct iovec iov = {
@@ -208,7 +208,7 @@ FileRead(File file, void *buffer, size_t amount, off_t offset,
 }
 
 static inline ssize_t
-FileWrite(File file, const void *buffer, size_t amount, off_t offset,
+FileWrite(File file, const void *buffer, size_t amount, pgoff_t offset,
 		  uint32 wait_event_info)
 {
 	struct iovec iov = {
diff --git a/src/port/win32pread.c b/src/port/win32pread.c
index 32d56c462e..18721cd7b2 100644
--- a/src/port/win32pread.c
+++ b/src/port/win32pread.c
@@ -17,7 +17,7 @@
 #include <windows.h>
 
 ssize_t
-pg_pread(int fd, void *buf, size_t size, off_t offset)
+pg_pread(int fd, void *buf, size_t size, pgoff_t offset)
 {
 	OVERLAPPED	overlapped = {0};
 	HANDLE		handle;
@@ -30,16 +30,16 @@ pg_pread(int fd, void *buf, size_t size, off_t offset)
 		return -1;
 	}
 
-	/* Avoid overflowing DWORD. */
+	/* Avoid overflowing DWORD */
 	size = Min(size, 1024 * 1024 * 1024);
+	/* Note that this changes the file position, despite not using it */
+	overlapped.Offset = (DWORD) offset;
+	overlapped.OffsetHigh = (DWORD) (offset >> 32);
 
-	/* Note that this changes the file position, despite not using it. */
-	overlapped.Offset = offset;
 	if (!ReadFile(handle, buf, size, &result, &overlapped))
 	{
 		if (GetLastError() == ERROR_HANDLE_EOF)
 			return 0;
-
 		_dosmaperr(GetLastError());
 		return -1;
 	}
diff --git a/src/port/win32pwrite.c b/src/port/win32pwrite.c
index 249aa6c468..d9a0d23c2b 100644
--- a/src/port/win32pwrite.c
+++ b/src/port/win32pwrite.c
@@ -15,9 +15,8 @@
 #include "c.h"
 
 #include <windows.h>
-
 ssize_t
-pg_pwrite(int fd, const void *buf, size_t size, off_t offset)
+pg_pwrite(int fd, const void *buf, size_t size, pgoff_t offset)
 {
 	OVERLAPPED	overlapped = {0};
 	HANDLE		handle;
@@ -30,11 +29,12 @@ pg_pwrite(int fd, const void *buf, size_t size, off_t offset)
 		return -1;
 	}
 
-	/* Avoid overflowing DWORD. */
+	/* Avoid overflowing DWORD */
 	size = Min(size, 1024 * 1024 * 1024);
 
-	/* Note that this changes the file position, despite not using it. */
-	overlapped.Offset = offset;
+	overlapped.Offset = (DWORD) offset;
+	overlapped.OffsetHigh = (DWORD) (offset >> 32);
+
 	if (!WriteFile(handle, buf, size, &result, &overlapped))
 	{
 		_dosmaperr(GetLastError());
diff --git a/src/test/modules/meson.build b/src/test/modules/meson.build
index 14fc761c4c..95af220a4d 100644
--- a/src/test/modules/meson.build
+++ b/src/test/modules/meson.build
@@ -28,6 +28,7 @@ subdir('test_ginpostinglist')
 subdir('test_int128')
 subdir('test_integerset')
 subdir('test_json_parser')
+subdir('test_large_files')
 subdir('test_lfind')
 subdir('test_lwlock_tranches')
 subdir('test_misc')
diff --git a/src/test/modules/test_large_files/Makefile b/src/test/modules/test_large_files/Makefile
new file mode 100644
index 0000000000..26bb53a51f
--- /dev/null
+++ b/src/test/modules/test_large_files/Makefile
@@ -0,0 +1,20 @@
+# src/test/modules/test_large_files/Makefile
+
+MODULE_big = test_large_files
+OBJS = test_large_files.o
+
+EXTENSION = test_large_files
+DATA = test_large_files--1.0.sql
+
+REGRESS = test_large_files
+
+ifdef USE_PGXS
+PG_CONFIG = pg_config
+PGXS := $(shell $(PG_CONFIG) --pgxs)
+include $(PGXS)
+else
+subdir = src/test/modules/test_large_files
+top_builddir = ../../../..
+include $(top_builddir)/src/Makefile.global
+include $(top_srcdir)/contrib/contrib-global.mk
+endif
diff --git a/src/test/modules/test_large_files/README b/src/test/modules/test_large_files/README
new file mode 100644
index 0000000000..d7caae49e6
--- /dev/null
+++ b/src/test/modules/test_large_files/README
@@ -0,0 +1,53 @@
+Test Module for Windows Large File I/O
+
+This test module provides functions to test PostgreSQL's ability to
+handle files larger than 4GB on Windows.
+
+Requirements
+
+- Windows platform
+- PostgreSQL built with segment size greater than 2GB
+- NTFS filesystem (for sparse file support)
+
+Functions
+
+test_create_sparse_file(filename text, size_gb int) RETURNS boolean
+
+Creates a sparse file of the specified size in gigabytes. This allows
+testing large offsets without actually writing gigabytes of data to
+disk.
+
+test_sparse_write_read(filename text, offset_gb float8, test_data text)
+RETURNS boolean
+
+Writes test data at the specified offset (in GB) using PostgreSQL's VFD
+layer (FileWrite), then reads it back using FileRead to verify basic I/O
+functionality.
+
+test_verify_offset_native(filename text, offset_gb float8, expected_data
+text) RETURNS boolean
+
+Critical for validation: Uses native Windows APIs (ReadFile with proper
+OVERLAPPED structure) to verify that data written by PostgreSQL is
+actually at the correct offset. This catches bugs where both write and
+read might use the same incorrect offset calculation (making a broken
+test appear to pass).
+
+Without this verification, a test could pass even with broken offset
+handling if both FileWrite and FileRead make the same mistake.
+
+What the Test Verifies
+
+1. Sparse file creation works on Windows
+2. PostgreSQL's FileWrite can write at offsets > 4GB
+3. PostgreSQL's FileRead can read from offsets > 4GB
+4. Data is actually at the correct offset (verified with native Windows
+   APIs)
+
+The native verification step is critical because without it, a test
+could pass even with broken offset handling. For example, if both
+FileWrite and FileRead truncate offsets to 32 bits, writing at 4.5GB
+would actually write at ~512MB, and reading at 4.5GB would read from
+~512MB - the test would find matching data but at the wrong location.
+The native verification catches this by independently checking the
+actual file offset.
diff --git a/src/test/modules/test_large_files/meson.build b/src/test/modules/test_large_files/meson.build
new file mode 100644
index 0000000000..c755e2cf16
--- /dev/null
+++ b/src/test/modules/test_large_files/meson.build
@@ -0,0 +1,29 @@
+# src/test/modules/test_large_files/meson.build
+
+test_large_files_sources = files(
+  'test_large_files.c',
+)
+
+if host_system == 'windows'
+  test_large_files = shared_module('test_large_files',
+    test_large_files_sources,
+    kwargs: pg_test_mod_args,
+  )
+  test_install_libs += test_large_files
+
+  test_install_data += files(
+    'test_large_files.control',
+    'test_large_files--1.0.sql',
+  )
+
+  tests += {
+    'name': 'test_large_files',
+    'sd': meson.current_source_dir(),
+    'bd': meson.current_build_dir(),
+    'tap': {
+      'tests': [
+        't/001_windows_large_files.pl',
+      ],
+    },
+  }
+endif
diff --git a/src/test/modules/test_large_files/t/001_windows_large_files.pl b/src/test/modules/test_large_files/t/001_windows_large_files.pl
new file mode 100644
index 0000000000..2fb0ef5e36
--- /dev/null
+++ b/src/test/modules/test_large_files/t/001_windows_large_files.pl
@@ -0,0 +1,65 @@
+#!/usr/bin/perl
+# Copyright (c) 2025, PostgreSQL Global Development Group
+
+=pod
+
+=head1 NAME
+
+001_windows_large_files.pl - Test Windows support for files >4GB
+
+=head1 SYNOPSIS
+
+  prove src/test/modules/test_large_files/t/001_windows_large_files.pl
+
+=head1 DESCRIPTION
+
+This test verifies that PostgreSQL on Windows can correctly handle file
+operations at offsets beyond 4GB. This requires PostgreSQL to be
+built with a segment size greater than 2GB.
+
+The test uses sparse files to avoid actually writing gigabytes of data.
+
+=cut
+
+use strict;
+use warnings;
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+use File::Spec;
+use File::Temp;
+
+if ($^O ne 'MSWin32')
+{
+	plan skip_all => 'test is Windows-specific';
+}
+
+plan tests => 4;
+
+my $node = PostgreSQL::Test::Cluster->new('main');
+$node->init;
+$node->start;
+
+$node->safe_psql('postgres', 'CREATE EXTENSION test_large_files;');
+pass("test_large_files extension loaded");
+
+my $tempdir = File::Temp->newdir();
+my $testfile = File::Spec->catfile($tempdir, 'large_file_test.dat');
+
+note "Test file: $testfile";
+
+my $create_result = $node->safe_psql('postgres',
+	"SELECT test_create_sparse_file('$testfile', 5);");
+is($create_result, 't', "Created 5GB sparse file");
+
+my $test_4_5gb = $node->safe_psql('postgres',
+	"SELECT test_sparse_write_read('$testfile', 4.5, 'TEST_DATA_AT_4.5GB');");
+is($test_4_5gb, 't', "Write/read successful at 4.5GB offset");
+
+my $verify_4_5gb = $node->safe_psql('postgres',
+	"SELECT test_verify_offset_native('$testfile', 4.5, 'TEST_DATA_AT_4.5GB');");
+is($verify_4_5gb, 't', "Native verification confirms data at correct 4.5GB offset");
+
+$node->stop;
+
+done_testing();
diff --git a/src/test/modules/test_large_files/test_large_files--1.0.sql b/src/test/modules/test_large_files/test_large_files--1.0.sql
new file mode 100644
index 0000000000..c4db84106c
--- /dev/null
+++ b/src/test/modules/test_large_files/test_large_files--1.0.sql
@@ -0,0 +1,36 @@
+-- src/test/modules/test_large_files/test_large_files--1.0.sql
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "CREATE EXTENSION test_large_files" to load this file. \quit
+
+--
+-- test_create_sparse_file(filename text, size_gb int) returns boolean
+--
+-- Creates a sparse file for testing. Windows only.
+--
+CREATE FUNCTION test_create_sparse_file(filename text, size_gb int)
+RETURNS boolean
+AS 'MODULE_PATHNAME', 'test_create_sparse_file'
+LANGUAGE C STRICT;
+
+--
+-- test_sparse_write_read(filename text, offset_gb numeric, test_data text) returns boolean
+--
+-- Writes data at a large offset and reads it back to verify correctness.
+-- Tests pg_pwrite/pg_pread with offsets beyond 2GB and 4GB. Windows only.
+--
+CREATE FUNCTION test_sparse_write_read(filename text, offset_gb float8, test_data text)
+RETURNS boolean
+AS 'MODULE_PATHNAME', 'test_sparse_write_read'
+LANGUAGE C STRICT;
+
+--
+-- test_verify_offset_native(filename text, offset_gb numeric, expected_data text) returns boolean
+--
+-- Uses native Windows APIs to verify data is at the correct offset.
+-- This ensures PostgreSQL's I/O didn't write to a wrapped/incorrect offset.
+--
+CREATE FUNCTION test_verify_offset_native(filename text, offset_gb float8, expected_data text)
+RETURNS boolean
+AS 'MODULE_PATHNAME', 'test_verify_offset_native'
+LANGUAGE C STRICT;
diff --git a/src/test/modules/test_large_files/test_large_files.c b/src/test/modules/test_large_files/test_large_files.c
new file mode 100644
index 0000000000..623d2d214c
--- /dev/null
+++ b/src/test/modules/test_large_files/test_large_files.c
@@ -0,0 +1,270 @@
+/* src/test/modules/test_large_files/test_large_files.c */
+
+#include "postgres.h"
+
+#include "fmgr.h"
+#include "storage/fd.h"
+#include "utils/builtins.h"
+
+#ifdef WIN32
+#include <windows.h>
+#include <winioctl.h>
+#endif
+
+PG_MODULE_MAGIC;
+
+PG_FUNCTION_INFO_V1(test_sparse_write_read);
+PG_FUNCTION_INFO_V1(test_create_sparse_file);
+PG_FUNCTION_INFO_V1(test_verify_offset_native);
+
+/*
+ * test_verify_offset_native(filename text, offset_gb numeric, expected_data text) returns boolean
+ *
+ * Uses native Windows APIs to read data at the specified offset and verify it matches.
+ * This ensures PostgreSQL's I/O functions wrote to the CORRECT offset, not a wrapped one.
+ * Windows only.
+ */
+Datum
+test_verify_offset_native(PG_FUNCTION_ARGS)
+{
+#ifdef WIN32
+	text	   *filename_text = PG_GETARG_TEXT_PP(0);
+	float8		offset_gb = PG_GETARG_FLOAT8(1);
+	text	   *expected_text = PG_GETARG_TEXT_PP(2);
+	char	   *filename;
+	char	   *expected_data;
+	char	   *read_buffer;
+	int			expected_len;
+	int64		offset;
+	HANDLE		hFile;
+	OVERLAPPED	overlapped = {0};
+	DWORD		bytesRead;
+	bool		success = false;
+
+	filename = text_to_cstring(filename_text);
+	expected_data = text_to_cstring(expected_text);
+	expected_len = strlen(expected_data) + 1;
+
+	/* Calculate offset in bytes */
+	offset = (int64) (offset_gb * 1024.0 * 1024.0 * 1024.0);
+
+	/* Open file with native Windows API */
+	hFile = CreateFile(filename,
+					   GENERIC_READ,
+					   FILE_SHARE_READ | FILE_SHARE_WRITE,
+					   NULL,
+					   OPEN_EXISTING,
+					   FILE_ATTRIBUTE_NORMAL,
+					   NULL);
+
+	if (hFile == INVALID_HANDLE_VALUE)
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not open file \"%s\" for verification: %lu",
+						filename, GetLastError())));
+
+	/* Set up OVERLAPPED structure with proper 64-bit offset */
+	overlapped.Offset = (DWORD)(offset & 0xFFFFFFFF);
+	overlapped.OffsetHigh = (DWORD)(offset >> 32);
+
+	/* Allocate read buffer */
+	read_buffer = palloc(expected_len);
+
+	/* Read using native Windows API */
+	if (!ReadFile(hFile, read_buffer, expected_len, &bytesRead, &overlapped))
+	{
+		DWORD error = GetLastError();
+		CloseHandle(hFile);
+		pfree(read_buffer);
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("native ReadFile failed at offset %lld: %lu",
+						offset, error)));
+	}
+
+	if (bytesRead != expected_len)
+	{
+		CloseHandle(hFile);
+		pfree(read_buffer);
+		ereport(ERROR,
+				(errmsg("native ReadFile read %lu bytes, expected %d",
+						bytesRead, expected_len)));
+	}
+
+	/* Verify data matches */
+	success = (memcmp(expected_data, read_buffer, expected_len) == 0);
+
+	pfree(read_buffer);
+	CloseHandle(hFile);
+
+	if (!success)
+		ereport(ERROR,
+				(errmsg("data mismatch at offset %lld: PostgreSQL wrote to wrong location",
+						offset)));
+
+	PG_RETURN_BOOL(success);
+#else
+	ereport(ERROR,
+			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+			 errmsg("this test is only supported on Windows")));
+	PG_RETURN_BOOL(false);
+#endif
+}
+
+/*
+ * test_create_sparse_file(filename text, size_gb int) returns boolean
+ *
+ * Creates a sparse file of the specified size in gigabytes.
+ * Windows only.
+ */
+Datum
+test_create_sparse_file(PG_FUNCTION_ARGS)
+{
+#ifdef WIN32
+	text	   *filename_text = PG_GETARG_TEXT_PP(0);
+	int32		size_gb = PG_GETARG_INT32(1);
+	char	   *filename;
+	HANDLE		hFile;
+	DWORD		bytesReturned;
+	LARGE_INTEGER fileSize;
+	bool		success = false;
+
+	filename = text_to_cstring(filename_text);
+
+	/* Open/create the file */
+	hFile = CreateFile(filename,
+					   GENERIC_WRITE,
+					   0,
+					   NULL,
+					   CREATE_ALWAYS,
+					   FILE_ATTRIBUTE_NORMAL,
+					   NULL);
+
+	if (hFile == INVALID_HANDLE_VALUE)
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not create file \"%s\": %lu",
+						filename, GetLastError())));
+
+	/* Mark as sparse */
+	if (!DeviceIoControl(hFile, FSCTL_SET_SPARSE, NULL, 0, NULL, 0,
+						 &bytesReturned, NULL))
+	{
+		CloseHandle(hFile);
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not set file sparse: %lu", GetLastError())));
+	}
+
+	/* Set file size */
+	fileSize.QuadPart = (int64) size_gb * 1024 * 1024 * 1024;
+	if (!SetFilePointerEx(hFile, fileSize, NULL, FILE_BEGIN))
+	{
+		CloseHandle(hFile);
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not set file pointer: %lu", GetLastError())));
+	}
+
+	if (!SetEndOfFile(hFile))
+	{
+		CloseHandle(hFile);
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not set end of file: %lu", GetLastError())));
+	}
+
+	success = true;
+	CloseHandle(hFile);
+
+	PG_RETURN_BOOL(success);
+#else
+	ereport(ERROR,
+			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+			 errmsg("sparse file test only supported on Windows")));
+	PG_RETURN_BOOL(false);
+#endif
+}
+
+/*
+ * test_sparse_write_read(filename text, offset_gb numeric, test_data text) returns boolean
+ *
+ * Writes test data at the specified offset (in GB) and reads it back to verify.
+ * Tests that pg_pwrite and pg_pread work correctly with large offsets.
+ * Windows only.
+ */
+Datum
+test_sparse_write_read(PG_FUNCTION_ARGS)
+{
+#ifdef WIN32
+	text	   *filename_text = PG_GETARG_TEXT_PP(0);
+	float8		offset_gb = PG_GETARG_FLOAT8(1);
+	text	   *test_data_text = PG_GETARG_TEXT_PP(2);
+	char	   *filename;
+	char	   *test_data;
+	char	   *read_buffer;
+	int			test_data_len;
+	pgoff_t		offset;
+	int			fd;
+	ssize_t		written;
+	ssize_t		nread;
+	bool		success = false;
+
+	filename = text_to_cstring(filename_text);
+	test_data = text_to_cstring(test_data_text);
+	test_data_len = strlen(test_data) + 1;	/* include null terminator */
+
+	/* Calculate offset in bytes */
+	offset = (pgoff_t) (offset_gb * 1024.0 * 1024.0 * 1024.0);
+
+	/* Open the file using PostgreSQL's VFD layer */
+	fd = BasicOpenFile(filename, O_RDWR | PG_BINARY);
+	if (fd < 0)
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not open file \"%s\": %m", filename)));
+
+	/* Write test data at the specified offset using pg_pwrite */
+	written = pg_pwrite(fd, test_data, test_data_len, offset);
+	if (written != test_data_len)
+	{
+		close(fd);
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not write to file at offset %lld: wrote %zd of %d bytes",
+						(long long) offset, written, test_data_len)));
+	}
+
+	/* Allocate buffer for reading */
+	read_buffer = palloc(test_data_len);
+
+	/* Read back the data using pg_pread */
+	nread = pg_pread(fd, read_buffer, test_data_len, offset);
+	if (nread != test_data_len)
+	{
+		close(fd);
+		pfree(read_buffer);
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not read from file at offset %lld: read %zd of %d bytes",
+						(long long) offset, nread, test_data_len)));
+	}
+
+	/* Verify data matches */
+	success = (memcmp(test_data, read_buffer, test_data_len) == 0);
+
+	pfree(read_buffer);
+	close(fd);
+
+	if (!success)
+		ereport(ERROR,
+				(errmsg("data mismatch: read data does not match written data")));
+
+	PG_RETURN_BOOL(success);
+#else
+	ereport(ERROR,
+			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+			 errmsg("this test is only supported on Windows")));
+	PG_RETURN_BOOL(false);
+#endif
+}
diff --git a/src/test/modules/test_large_files/test_large_files.control b/src/test/modules/test_large_files/test_large_files.control
new file mode 100644
index 0000000000..9b0a30974b
--- /dev/null
+++ b/src/test/modules/test_large_files/test_large_files.control
@@ -0,0 +1,5 @@
+# test_large_files extension
+comment = 'Test module for large file I/O on Windows'
+default_version = '1.0'
+module_pathname = '$libdir/test_large_files'
+relocatable = true
-- 
2.46.0.windows.1

From 88e1d4c56f74c604d3773da4c45c0c592724adc5 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Sat, 8 Nov 2025 18:36:52 +0900
Subject: [PATCH] Add check for large files in meson.build

This check existed in the MSVC scripts that have been removed in v16,
and was missing from meson.

Backpatch-through: 16
---
 meson.build | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/meson.build b/meson.build
index 00c46d400714..c7bc2df2c82f 100644
--- a/meson.build
+++ b/meson.build
@@ -452,6 +452,14 @@ else
   segsize = (get_option('segsize') * 1024 * 1024 * 1024) / blocksize
 endif
 
+# If we don't have largefile support, can't handle segment size >= 2GB.
+if cc.sizeof('off_t', args: test_c_args) < 8
+  segsize_bytes = segsize * blocksize
+  if segsize_bytes >= (2 * 1024 * 1024 * 1024)
+    error('Large file support is not enabled. Segment size cannot be larger than 1GB.')
+  endif
+endif
+
 cdata.set('BLCKSZ', blocksize, description:
 '''Size of a disk block --- this also limits the size of a tuple. You can set
    it bigger if you need bigger tuples (although TOAST should reduce the need
-- 
2.51.0

From c46c56e7b86e26a63f9b0b638d44558f2af93b8d Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Thu, 13 Nov 2025 12:59:56 +0900
Subject: [PATCH v5] Add test module test_large_files

---
 src/test/modules/Makefile                     |   1 +
 src/test/modules/meson.build                  |   1 +
 src/test/modules/test_large_files/Makefile    |  19 ++
 src/test/modules/test_large_files/README      |  53 ++++
 src/test/modules/test_large_files/meson.build |  29 ++
 .../t/001_windows_large_files.pl              |  65 +++++
 .../test_large_files--1.0.sql                 |  36 +++
 .../test_large_files/test_large_files.c       | 270 ++++++++++++++++++
 .../test_large_files/test_large_files.control |   5 +
 .../log/regress_log_001_windows_large_files   |   1 +
 10 files changed, 480 insertions(+)
 create mode 100644 src/test/modules/test_large_files/Makefile
 create mode 100644 src/test/modules/test_large_files/README
 create mode 100644 src/test/modules/test_large_files/meson.build
 create mode 100644 src/test/modules/test_large_files/t/001_windows_large_files.pl
 create mode 100644 src/test/modules/test_large_files/test_large_files--1.0.sql
 create mode 100644 src/test/modules/test_large_files/test_large_files.c
 create mode 100644 src/test/modules/test_large_files/test_large_files.control
 create mode 100644 src/test/modules/test_large_files/tmp_check/log/regress_log_001_windows_large_files

diff --git a/src/test/modules/Makefile b/src/test/modules/Makefile
index 902a79541010..442713428791 100644
--- a/src/test/modules/Makefile
+++ b/src/test/modules/Makefile
@@ -29,6 +29,7 @@ SUBDIRS = \
 		  test_int128 \
 		  test_integerset \
 		  test_json_parser \
+		  test_large_files \
 		  test_lfind \
 		  test_lwlock_tranches \
 		  test_misc \
diff --git a/src/test/modules/meson.build b/src/test/modules/meson.build
index 14fc761c4cfa..95af220a4d97 100644
--- a/src/test/modules/meson.build
+++ b/src/test/modules/meson.build
@@ -28,6 +28,7 @@ subdir('test_ginpostinglist')
 subdir('test_int128')
 subdir('test_integerset')
 subdir('test_json_parser')
+subdir('test_large_files')
 subdir('test_lfind')
 subdir('test_lwlock_tranches')
 subdir('test_misc')
diff --git a/src/test/modules/test_large_files/Makefile b/src/test/modules/test_large_files/Makefile
new file mode 100644
index 000000000000..f9fa977797d0
--- /dev/null
+++ b/src/test/modules/test_large_files/Makefile
@@ -0,0 +1,19 @@
+# src/test/modules/test_large_files/Makefile
+
+MODULES = test_large_files
+
+EXTENSION = test_large_files
+DATA = test_large_files--1.0.sql
+
+TAP_TESTS = 1
+
+ifdef USE_PGXS
+PG_CONFIG = pg_config
+PGXS := $(shell $(PG_CONFIG) --pgxs)
+include $(PGXS)
+else
+subdir = src/test/modules/test_large_files
+top_builddir = ../../../..
+include $(top_builddir)/src/Makefile.global
+include $(top_srcdir)/contrib/contrib-global.mk
+endif
diff --git a/src/test/modules/test_large_files/README b/src/test/modules/test_large_files/README
new file mode 100644
index 000000000000..d7caae49e6a3
--- /dev/null
+++ b/src/test/modules/test_large_files/README
@@ -0,0 +1,53 @@
+Test Module for Windows Large File I/O
+
+This test module provides functions to test PostgreSQL's ability to
+handle files larger than 4GB on Windows.
+
+Requirements
+
+- Windows platform
+- PostgreSQL built with segment size greater than 2GB
+- NTFS filesystem (for sparse file support)
+
+Functions
+
+test_create_sparse_file(filename text, size_gb int) RETURNS boolean
+
+Creates a sparse file of the specified size in gigabytes. This allows
+testing large offsets without actually writing gigabytes of data to
+disk.
+
+test_sparse_write_read(filename text, offset_gb float8, test_data text)
+RETURNS boolean
+
+Writes test data at the specified offset (in GB) using PostgreSQL's VFD
+layer (FileWrite), then reads it back using FileRead to verify basic I/O
+functionality.
+
+test_verify_offset_native(filename text, offset_gb float8, expected_data
+text) RETURNS boolean
+
+Critical for validation: Uses native Windows APIs (ReadFile with proper
+OVERLAPPED structure) to verify that data written by PostgreSQL is
+actually at the correct offset. This catches bugs where both write and
+read might use the same incorrect offset calculation (making a broken
+test appear to pass).
+
+Without this verification, a test could pass even with broken offset
+handling if both FileWrite and FileRead make the same mistake.
+
+What the Test Verifies
+
+1. Sparse file creation works on Windows
+2. PostgreSQL's FileWrite can write at offsets > 4GB
+3. PostgreSQL's FileRead can read from offsets > 4GB
+4. Data is actually at the correct offset (verified with native Windows
+   APIs)
+
+The native verification step is critical because without it, a test
+could pass even with broken offset handling. For example, if both
+FileWrite and FileRead truncate offsets to 32 bits, writing at 4.5GB
+would actually write at ~512MB, and reading at 4.5GB would read from
+~512MB - the test would find matching data but at the wrong location.
+The native verification catches this by independently checking the
+actual file offset.
diff --git a/src/test/modules/test_large_files/meson.build b/src/test/modules/test_large_files/meson.build
new file mode 100644
index 000000000000..c755e2cf16d0
--- /dev/null
+++ b/src/test/modules/test_large_files/meson.build
@@ -0,0 +1,29 @@
+# src/test/modules/test_large_files/meson.build
+
+test_large_files_sources = files(
+  'test_large_files.c',
+)
+
+if host_system == 'windows'
+  test_large_files = shared_module('test_large_files',
+    test_large_files_sources,
+    kwargs: pg_test_mod_args,
+  )
+  test_install_libs += test_large_files
+
+  test_install_data += files(
+    'test_large_files.control',
+    'test_large_files--1.0.sql',
+  )
+
+  tests += {
+    'name': 'test_large_files',
+    'sd': meson.current_source_dir(),
+    'bd': meson.current_build_dir(),
+    'tap': {
+      'tests': [
+        't/001_windows_large_files.pl',
+      ],
+    },
+  }
+endif
diff --git a/src/test/modules/test_large_files/t/001_windows_large_files.pl b/src/test/modules/test_large_files/t/001_windows_large_files.pl
new file mode 100644
index 000000000000..2fb0ef5e36bf
--- /dev/null
+++ b/src/test/modules/test_large_files/t/001_windows_large_files.pl
@@ -0,0 +1,65 @@
+#!/usr/bin/perl
+# Copyright (c) 2025, PostgreSQL Global Development Group
+
+=pod
+
+=head1 NAME
+
+001_windows_large_files.pl - Test Windows support for files >4GB
+
+=head1 SYNOPSIS
+
+  prove src/test/modules/test_large_files/t/001_windows_large_files.pl
+
+=head1 DESCRIPTION
+
+This test verifies that PostgreSQL on Windows can correctly handle file
+operations at offsets beyond 4GB. This requires PostgreSQL to be
+built with a segment size greater than 2GB.
+
+The test uses sparse files to avoid actually writing gigabytes of data.
+
+=cut
+
+use strict;
+use warnings;
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+use File::Spec;
+use File::Temp;
+
+if ($^O ne 'MSWin32')
+{
+	plan skip_all => 'test is Windows-specific';
+}
+
+plan tests => 4;
+
+my $node = PostgreSQL::Test::Cluster->new('main');
+$node->init;
+$node->start;
+
+$node->safe_psql('postgres', 'CREATE EXTENSION test_large_files;');
+pass("test_large_files extension loaded");
+
+my $tempdir = File::Temp->newdir();
+my $testfile = File::Spec->catfile($tempdir, 'large_file_test.dat');
+
+note "Test file: $testfile";
+
+my $create_result = $node->safe_psql('postgres',
+	"SELECT test_create_sparse_file('$testfile', 5);");
+is($create_result, 't', "Created 5GB sparse file");
+
+my $test_4_5gb = $node->safe_psql('postgres',
+	"SELECT test_sparse_write_read('$testfile', 4.5, 'TEST_DATA_AT_4.5GB');");
+is($test_4_5gb, 't', "Write/read successful at 4.5GB offset");
+
+my $verify_4_5gb = $node->safe_psql('postgres',
+	"SELECT test_verify_offset_native('$testfile', 4.5, 'TEST_DATA_AT_4.5GB');");
+is($verify_4_5gb, 't', "Native verification confirms data at correct 4.5GB offset");
+
+$node->stop;
+
+done_testing();
diff --git a/src/test/modules/test_large_files/test_large_files--1.0.sql b/src/test/modules/test_large_files/test_large_files--1.0.sql
new file mode 100644
index 000000000000..c4db84106c8d
--- /dev/null
+++ b/src/test/modules/test_large_files/test_large_files--1.0.sql
@@ -0,0 +1,36 @@
+-- src/test/modules/test_large_files/test_large_files--1.0.sql
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "CREATE EXTENSION test_large_files" to load this file. \quit
+
+--
+-- test_create_sparse_file(filename text, size_gb int) returns boolean
+--
+-- Creates a sparse file for testing. Windows only.
+--
+CREATE FUNCTION test_create_sparse_file(filename text, size_gb int)
+RETURNS boolean
+AS 'MODULE_PATHNAME', 'test_create_sparse_file'
+LANGUAGE C STRICT;
+
+--
+-- test_sparse_write_read(filename text, offset_gb numeric, test_data text) returns boolean
+--
+-- Writes data at a large offset and reads it back to verify correctness.
+-- Tests pg_pwrite/pg_pread with offsets beyond 2GB and 4GB. Windows only.
+--
+CREATE FUNCTION test_sparse_write_read(filename text, offset_gb float8, test_data text)
+RETURNS boolean
+AS 'MODULE_PATHNAME', 'test_sparse_write_read'
+LANGUAGE C STRICT;
+
+--
+-- test_verify_offset_native(filename text, offset_gb numeric, expected_data text) returns boolean
+--
+-- Uses native Windows APIs to verify data is at the correct offset.
+-- This ensures PostgreSQL's I/O didn't write to a wrapped/incorrect offset.
+--
+CREATE FUNCTION test_verify_offset_native(filename text, offset_gb float8, expected_data text)
+RETURNS boolean
+AS 'MODULE_PATHNAME', 'test_verify_offset_native'
+LANGUAGE C STRICT;
diff --git a/src/test/modules/test_large_files/test_large_files.c b/src/test/modules/test_large_files/test_large_files.c
new file mode 100644
index 000000000000..623d2d214cde
--- /dev/null
+++ b/src/test/modules/test_large_files/test_large_files.c
@@ -0,0 +1,270 @@
+/* src/test/modules/test_large_files/test_large_files.c */
+
+#include "postgres.h"
+
+#include "fmgr.h"
+#include "storage/fd.h"
+#include "utils/builtins.h"
+
+#ifdef WIN32
+#include <windows.h>
+#include <winioctl.h>
+#endif
+
+PG_MODULE_MAGIC;
+
+PG_FUNCTION_INFO_V1(test_sparse_write_read);
+PG_FUNCTION_INFO_V1(test_create_sparse_file);
+PG_FUNCTION_INFO_V1(test_verify_offset_native);
+
+/*
+ * test_verify_offset_native(filename text, offset_gb numeric, expected_data text) returns boolean
+ *
+ * Uses native Windows APIs to read data at the specified offset and verify it matches.
+ * This ensures PostgreSQL's I/O functions wrote to the CORRECT offset, not a wrapped one.
+ * Windows only.
+ */
+Datum
+test_verify_offset_native(PG_FUNCTION_ARGS)
+{
+#ifdef WIN32
+	text	   *filename_text = PG_GETARG_TEXT_PP(0);
+	float8		offset_gb = PG_GETARG_FLOAT8(1);
+	text	   *expected_text = PG_GETARG_TEXT_PP(2);
+	char	   *filename;
+	char	   *expected_data;
+	char	   *read_buffer;
+	int			expected_len;
+	int64		offset;
+	HANDLE		hFile;
+	OVERLAPPED	overlapped = {0};
+	DWORD		bytesRead;
+	bool		success = false;
+
+	filename = text_to_cstring(filename_text);
+	expected_data = text_to_cstring(expected_text);
+	expected_len = strlen(expected_data) + 1;
+
+	/* Calculate offset in bytes */
+	offset = (int64) (offset_gb * 1024.0 * 1024.0 * 1024.0);
+
+	/* Open file with native Windows API */
+	hFile = CreateFile(filename,
+					   GENERIC_READ,
+					   FILE_SHARE_READ | FILE_SHARE_WRITE,
+					   NULL,
+					   OPEN_EXISTING,
+					   FILE_ATTRIBUTE_NORMAL,
+					   NULL);
+
+	if (hFile == INVALID_HANDLE_VALUE)
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not open file \"%s\" for verification: %lu",
+						filename, GetLastError())));
+
+	/* Set up OVERLAPPED structure with proper 64-bit offset */
+	overlapped.Offset = (DWORD)(offset & 0xFFFFFFFF);
+	overlapped.OffsetHigh = (DWORD)(offset >> 32);
+
+	/* Allocate read buffer */
+	read_buffer = palloc(expected_len);
+
+	/* Read using native Windows API */
+	if (!ReadFile(hFile, read_buffer, expected_len, &bytesRead, &overlapped))
+	{
+		DWORD error = GetLastError();
+		CloseHandle(hFile);
+		pfree(read_buffer);
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("native ReadFile failed at offset %lld: %lu",
+						offset, error)));
+	}
+
+	if (bytesRead != expected_len)
+	{
+		CloseHandle(hFile);
+		pfree(read_buffer);
+		ereport(ERROR,
+				(errmsg("native ReadFile read %lu bytes, expected %d",
+						bytesRead, expected_len)));
+	}
+
+	/* Verify data matches */
+	success = (memcmp(expected_data, read_buffer, expected_len) == 0);
+
+	pfree(read_buffer);
+	CloseHandle(hFile);
+
+	if (!success)
+		ereport(ERROR,
+				(errmsg("data mismatch at offset %lld: PostgreSQL wrote to wrong location",
+						offset)));
+
+	PG_RETURN_BOOL(success);
+#else
+	ereport(ERROR,
+			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+			 errmsg("this test is only supported on Windows")));
+	PG_RETURN_BOOL(false);
+#endif
+}
+
+/*
+ * test_create_sparse_file(filename text, size_gb int) returns boolean
+ *
+ * Creates a sparse file of the specified size in gigabytes.
+ * Windows only.
+ */
+Datum
+test_create_sparse_file(PG_FUNCTION_ARGS)
+{
+#ifdef WIN32
+	text	   *filename_text = PG_GETARG_TEXT_PP(0);
+	int32		size_gb = PG_GETARG_INT32(1);
+	char	   *filename;
+	HANDLE		hFile;
+	DWORD		bytesReturned;
+	LARGE_INTEGER fileSize;
+	bool		success = false;
+
+	filename = text_to_cstring(filename_text);
+
+	/* Open/create the file */
+	hFile = CreateFile(filename,
+					   GENERIC_WRITE,
+					   0,
+					   NULL,
+					   CREATE_ALWAYS,
+					   FILE_ATTRIBUTE_NORMAL,
+					   NULL);
+
+	if (hFile == INVALID_HANDLE_VALUE)
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not create file \"%s\": %lu",
+						filename, GetLastError())));
+
+	/* Mark as sparse */
+	if (!DeviceIoControl(hFile, FSCTL_SET_SPARSE, NULL, 0, NULL, 0,
+						 &bytesReturned, NULL))
+	{
+		CloseHandle(hFile);
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not set file sparse: %lu", GetLastError())));
+	}
+
+	/* Set file size */
+	fileSize.QuadPart = (int64) size_gb * 1024 * 1024 * 1024;
+	if (!SetFilePointerEx(hFile, fileSize, NULL, FILE_BEGIN))
+	{
+		CloseHandle(hFile);
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not set file pointer: %lu", GetLastError())));
+	}
+
+	if (!SetEndOfFile(hFile))
+	{
+		CloseHandle(hFile);
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not set end of file: %lu", GetLastError())));
+	}
+
+	success = true;
+	CloseHandle(hFile);
+
+	PG_RETURN_BOOL(success);
+#else
+	ereport(ERROR,
+			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+			 errmsg("sparse file test only supported on Windows")));
+	PG_RETURN_BOOL(false);
+#endif
+}
+
+/*
+ * test_sparse_write_read(filename text, offset_gb numeric, test_data text) returns boolean
+ *
+ * Writes test data at the specified offset (in GB) and reads it back to verify.
+ * Tests that pg_pwrite and pg_pread work correctly with large offsets.
+ * Windows only.
+ */
+Datum
+test_sparse_write_read(PG_FUNCTION_ARGS)
+{
+#ifdef WIN32
+	text	   *filename_text = PG_GETARG_TEXT_PP(0);
+	float8		offset_gb = PG_GETARG_FLOAT8(1);
+	text	   *test_data_text = PG_GETARG_TEXT_PP(2);
+	char	   *filename;
+	char	   *test_data;
+	char	   *read_buffer;
+	int			test_data_len;
+	pgoff_t		offset;
+	int			fd;
+	ssize_t		written;
+	ssize_t		nread;
+	bool		success = false;
+
+	filename = text_to_cstring(filename_text);
+	test_data = text_to_cstring(test_data_text);
+	test_data_len = strlen(test_data) + 1;	/* include null terminator */
+
+	/* Calculate offset in bytes */
+	offset = (pgoff_t) (offset_gb * 1024.0 * 1024.0 * 1024.0);
+
+	/* Open the file using PostgreSQL's VFD layer */
+	fd = BasicOpenFile(filename, O_RDWR | PG_BINARY);
+	if (fd < 0)
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not open file \"%s\": %m", filename)));
+
+	/* Write test data at the specified offset using pg_pwrite */
+	written = pg_pwrite(fd, test_data, test_data_len, offset);
+	if (written != test_data_len)
+	{
+		close(fd);
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not write to file at offset %lld: wrote %zd of %d bytes",
+						(long long) offset, written, test_data_len)));
+	}
+
+	/* Allocate buffer for reading */
+	read_buffer = palloc(test_data_len);
+
+	/* Read back the data using pg_pread */
+	nread = pg_pread(fd, read_buffer, test_data_len, offset);
+	if (nread != test_data_len)
+	{
+		close(fd);
+		pfree(read_buffer);
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not read from file at offset %lld: read %zd of %d bytes",
+						(long long) offset, nread, test_data_len)));
+	}
+
+	/* Verify data matches */
+	success = (memcmp(test_data, read_buffer, test_data_len) == 0);
+
+	pfree(read_buffer);
+	close(fd);
+
+	if (!success)
+		ereport(ERROR,
+				(errmsg("data mismatch: read data does not match written data")));
+
+	PG_RETURN_BOOL(success);
+#else
+	ereport(ERROR,
+			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+			 errmsg("this test is only supported on Windows")));
+	PG_RETURN_BOOL(false);
+#endif
+}
diff --git a/src/test/modules/test_large_files/test_large_files.control b/src/test/modules/test_large_files/test_large_files.control
new file mode 100644
index 000000000000..9b0a30974b95
--- /dev/null
+++ b/src/test/modules/test_large_files/test_large_files.control
@@ -0,0 +1,5 @@
+# test_large_files extension
+comment = 'Test module for large file I/O on Windows'
+default_version = '1.0'
+module_pathname = '$libdir/test_large_files'
+relocatable = true
diff --git a/src/test/modules/test_large_files/tmp_check/log/regress_log_001_windows_large_files b/src/test/modules/test_large_files/tmp_check/log/regress_log_001_windows_large_files
new file mode 100644
index 000000000000..6d1526ee93d9
--- /dev/null
+++ b/src/test/modules/test_large_files/tmp_check/log/regress_log_001_windows_large_files
@@ -0,0 +1 @@
+[12:57:48.543](0.006s) 1..0 # SKIP test is Windows-specific
-- 
2.51.0

From 4513158e1ecd912873628b88191b15d24846cca2 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Thu, 13 Nov 2025 12:59:56 +0900
Subject: [PATCH v6] Fix Windows file I/O for offsets beyond 2GB

Two bugs prevented files from exceeding 2GB on Windows when built with
segment sizes larger than the default 1GB.

First, off_t is only 32 bits on Windows with MSVC, causing signed
overflow at 2GB. Change the file I/O layer to use pgoff_t consistently:
fd.c, md.c, pg_iovec.h, file_utils.c, and their associated headers.
This is safe on Unix where pgoff_t equals off_t.

Second, the Windows pg_pwrite() and pg_pread() implementations only set
the low 32 bits of the OVERLAPPED structure, leaving OffsetHigh at zero.
This happens to work below 4GB but wraps around above that. Set both
Offset and OffsetHigh properly.

Add a regression test that validates I/O at 4GB+1. Testing beyond 4GB is
necessary because OffsetHigh is naturally zero at smaller offsets and
the bug would pass unnoticed. The test uses FileSize() to independently
verify that writes reach the correct location.
---
 meson.build                                   |   8 --
 src/test/modules/Makefile                     |   1 +
 src/test/modules/meson.build                  |   1 +
 src/test/modules/test_large_files/Makefile    |  22 ++++
 src/test/modules/test_large_files/README      |  74 +++++++++++
 .../expected/test_large_files.out             |  19 +++
 src/test/modules/test_large_files/meson.build |  36 ++++++
 .../test_large_files/sql/test_large_files.sql |  14 +++
 .../test_large_files--1.0.sql                 |   9 ++
 .../test_large_files/test_large_files.c       | 117 ++++++++++++++++++
 .../test_large_files/test_large_files.control |   5 +
 11 files changed, 298 insertions(+), 8 deletions(-)
 create mode 100644 src/test/modules/test_large_files/Makefile
 create mode 100644 src/test/modules/test_large_files/README
 create mode 100644 src/test/modules/test_large_files/expected/test_large_files.out
 create mode 100644 src/test/modules/test_large_files/meson.build
 create mode 100644 src/test/modules/test_large_files/sql/test_large_files.sql
 create mode 100644 src/test/modules/test_large_files/test_large_files--1.0.sql
 create mode 100644 src/test/modules/test_large_files/test_large_files.c
 create mode 100644 src/test/modules/test_large_files/test_large_files.control

diff --git a/meson.build b/meson.build
index 6e7ddd7468..b8f64c176d 100644
--- a/meson.build
+++ b/meson.build
@@ -452,14 +452,6 @@ else
   segsize = (get_option('segsize') * 1024 * 1024 * 1024) / blocksize
 endif
 
-# If we don't have largefile support, can't handle segment size >= 2GB.
-if cc.sizeof('off_t', args: test_c_args) < 8
-  segsize_bytes = segsize * blocksize
-  if segsize_bytes >= (2 * 1024 * 1024 * 1024)
-    error('Large file support is not enabled. Segment size cannot be larger than 1GB.')
-  endif
-endif
-
 cdata.set('BLCKSZ', blocksize, description:
 '''Size of a disk block --- this also limits the size of a tuple. You can set
    it bigger if you need bigger tuples (although TOAST should reduce the need
diff --git a/src/test/modules/Makefile b/src/test/modules/Makefile
index d079b91b1a..a045065ad9 100644
--- a/src/test/modules/Makefile
+++ b/src/test/modules/Makefile
@@ -30,6 +30,7 @@ SUBDIRS = \
 		  test_int128 \
 		  test_integerset \
 		  test_json_parser \
+		  test_large_files \
 		  test_lfind \
 		  test_lwlock_tranches \
 		  test_misc \
diff --git a/src/test/modules/meson.build b/src/test/modules/meson.build
index f5114469b9..9888009720 100644
--- a/src/test/modules/meson.build
+++ b/src/test/modules/meson.build
@@ -29,6 +29,7 @@ subdir('test_ginpostinglist')
 subdir('test_int128')
 subdir('test_integerset')
 subdir('test_json_parser')
+subdir('test_large_files')
 subdir('test_lfind')
 subdir('test_lwlock_tranches')
 subdir('test_misc')
diff --git a/src/test/modules/test_large_files/Makefile b/src/test/modules/test_large_files/Makefile
new file mode 100644
index 0000000000..1960e31e52
--- /dev/null
+++ b/src/test/modules/test_large_files/Makefile
@@ -0,0 +1,22 @@
+MODULE_big = test_large_files
+OBJS = \
+	$(WIN32RES) \
+	test_large_files.o
+
+EXTENSION = test_large_files
+DATA = test_large_files--1.0.sql
+
+REGRESS = test_large_files
+
+NO_INSTALLCHECK = 1
+
+ifdef USE_PGXS
+PG_CONFIG = pg_config
+PGXS := $(shell $(PG_CONFIG) --pgxs)
+include $(PGXS)
+else
+subdir = src/test/modules/test_large_files
+top_builddir = ../../../..
+include $(top_builddir)/src/Makefile.global
+include $(top_srcdir)/contrib/contrib-global.mk
+endif
diff --git a/src/test/modules/test_large_files/README b/src/test/modules/test_large_files/README
new file mode 100644
index 0000000000..467ec629fd
--- /dev/null
+++ b/src/test/modules/test_large_files/README
@@ -0,0 +1,74 @@
+test_large_files
+================
+
+This test module validates PostgreSQL's handling of files larger than
+4GB, specifically testing that pgoff_t (64-bit file offset type) is
+used correctly throughout the file I/O layer.
+
+Background
+----------
+
+On Windows with MSVC, off_t is only 32 bits, causing signed integer
+overflow at 2GB (2^31 bytes). Additionally, Windows' OVERLAPPED structure
+requires both low and high 32-bit offset fields to be set for offsets
+beyond 4GB. PostgreSQL defines pgoff_t as a 64-bit type (__int64 on
+Windows, off_t on Unix where it's already 64-bit) to handle large files
+correctly.
+
+Two bugs were fixed:
+
+1. Pervasive use of off_t where pgoff_t should be used in fd.c, md.c,
+   and related file I/O functions. This caused failures at exactly 2GB.
+
+2. Windows-specific bug in pg_pwrite()/pg_pread() where the OVERLAPPED
+   structure only set the low 32 bits of the file offset (Offset field),
+   leaving OffsetHigh at zero, causing wrap-around at 4GB.
+
+Test Design
+-----------
+
+The test validates file I/O at 4GB + 1 byte:
+
+1. Writes "OFFSET_0" at byte 0
+2. Writes "TESTDATA" at byte 4GB+1
+3. Checks FileSize() reports ~4GB (not ~16 bytes from wrap-around)
+4. Reads offset 0 to verify it wasn't corrupted by wrap-around
+5. Reads offset 4GB+1 to verify data integrity
+
+This approach catches both bugs:
+- The off_t truncation bug (would fail at 2GB writes)
+- The OVERLAPPED OffsetHigh bug (only manifests at 4GB+ where high bits != 0)
+
+Testing at 4GB+1 is critical because at 2GB+1, OffsetHigh would naturally
+be zero, so bugs in setting OffsetHigh wouldn't be detected. At 4GB+1,
+OffsetHigh must be 1, so the test verifies it's set correctly.
+
+The test catches the bug even if both read and write have the same
+truncation issue, because FileSize() provides independent verification.
+
+The test only runs on platforms with 64-bit pgoff_t (checked via
+sizeof(pgoff_t) >= 8).
+
+Platform Support
+----------------
+
+- Linux/Unix: Automatically creates sparse files (fast, no disk space used)
+- Windows NTFS: Creates sparse file efficiently
+- 32-bit offset platforms: Test is skipped automatically
+
+Running the Test
+----------------
+
+The test only runs during 'make check' or 'meson test', not on
+'make installcheck'. This is intentional, as the test creates temporary
+files and is designed for development/CI testing rather than production
+validation.
+
+  make check
+
+or with meson:
+
+  meson test test_large_files
+
+The test completes in seconds on most platforms. On Windows, the test
+may take longer as the OS allocates the sparse file structure.
diff --git a/src/test/modules/test_large_files/expected/test_large_files.out b/src/test/modules/test_large_files/expected/test_large_files.out
new file mode 100644
index 0000000000..a2128cdd8d
--- /dev/null
+++ b/src/test/modules/test_large_files/expected/test_large_files.out
@@ -0,0 +1,19 @@
+CREATE EXTENSION test_large_files;
+SELECT test_large_files_offset_size() >= 8 AS has_large_file_support \gset
+-- Only run test on platforms with 64-bit offsets
+\if :has_large_file_support
+    -- Test file I/O at 4GB + 1 byte boundary
+    -- This validates that pgoff_t is used correctly throughout
+    -- the file I/O layer and catches both:
+    -- 1. off_t truncation bugs (affects all operations at 2GB+)
+    -- 2. Windows OVERLAPPED structure bugs (OffsetHigh must be set
+    --    correctly at 4GB+ where high 32 bits are non-zero)
+    SELECT test_large_files_test_4gb_boundary();
+ test_large_files_test_4gb_boundary 
+------------------------------------
+ 4GB boundary test passed
+(1 row)
+
+\else
+    SELECT 'Skipped - 32-bit offsets not supported'::text AS test_large_files_test_4gb_boundary;
+\endif
diff --git a/src/test/modules/test_large_files/meson.build b/src/test/modules/test_large_files/meson.build
new file mode 100644
index 0000000000..2110bcf23f
--- /dev/null
+++ b/src/test/modules/test_large_files/meson.build
@@ -0,0 +1,36 @@
+# src/test/modules/test_large_files/meson.build
+
+test_large_files_sources = files(
+  'test_large_files.c',
+)
+
+if host_system == 'windows'
+  test_large_files_sources += rc_lib_gen.process(win32ver_rc, extra_args: [
+    '--NAME', 'test_large_files',
+    '--FILEDESC', 'test_large_files - test module for large file I/O',])
+endif
+
+test_large_files = shared_module('test_large_files',
+  test_large_files_sources,
+  kwargs: pg_test_mod_args,
+)
+test_install_libs += test_large_files
+
+test_install_data += files(
+  'test_large_files.control',
+  'test_large_files--1.0.sql',
+)
+
+tests += {
+  'name': 'test_large_files',
+  'sd': meson.current_source_dir(),
+  'bd': meson.current_build_dir(),
+  'regress': {
+    'sql': [
+      'test_large_files',
+    ],
+    # Don't run on installcheck - only during regular check
+    'regress_args': ['--no-locale'],
+    'runningcheck': false,
+  },
+}
diff --git a/src/test/modules/test_large_files/sql/test_large_files.sql b/src/test/modules/test_large_files/sql/test_large_files.sql
new file mode 100644
index 0000000000..4543b357f6
--- /dev/null
+++ b/src/test/modules/test_large_files/sql/test_large_files.sql
@@ -0,0 +1,14 @@
+CREATE EXTENSION test_large_files;
+SELECT test_large_files_offset_size() >= 8 AS has_large_file_support \gset
+-- Only run test on platforms with 64-bit offsets
+\if :has_large_file_support
+    -- Test file I/O at 4GB + 1 byte boundary
+    -- This validates that pgoff_t is used correctly throughout
+    -- the file I/O layer and catches both:
+    -- 1. off_t truncation bugs (affects all operations at 2GB+)
+    -- 2. Windows OVERLAPPED structure bugs (OffsetHigh must be set
+    --    correctly at 4GB+ where high 32 bits are non-zero)
+    SELECT test_large_files_test_4gb_boundary();
+\else
+    SELECT 'Skipped - 32-bit offsets not supported'::text AS test_large_files_test_4gb_boundary;
+\endif
diff --git a/src/test/modules/test_large_files/test_large_files--1.0.sql b/src/test/modules/test_large_files/test_large_files--1.0.sql
new file mode 100644
index 0000000000..9b13c398a1
--- /dev/null
+++ b/src/test/modules/test_large_files/test_large_files--1.0.sql
@@ -0,0 +1,9 @@
+CREATE FUNCTION test_large_files_offset_size()
+RETURNS integer
+AS 'MODULE_PATHNAME'
+LANGUAGE C STRICT;
+
+CREATE FUNCTION test_large_files_test_4gb_boundary()
+RETURNS text
+AS 'MODULE_PATHNAME'
+LANGUAGE C STRICT;
diff --git a/src/test/modules/test_large_files/test_large_files.c b/src/test/modules/test_large_files/test_large_files.c
new file mode 100644
index 0000000000..70e675d473
--- /dev/null
+++ b/src/test/modules/test_large_files/test_large_files.c
@@ -0,0 +1,117 @@
+/*-------------------------------------------------------------------------
+ *
+ * test_large_files.c
+ *		Test module for large file I/O operations
+ *
+ * This module tests PostgreSQL's ability to handle file offsets larger
+ * than 2GB (2^31 bytes), validating that pgoff_t is correctly used
+ * throughout the file I/O layer.
+ *
+ * Copyright (c) 2025, PostgreSQL Global Development Group
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include "fmgr.h"
+#include "miscadmin.h"
+#include "storage/fd.h"
+#include "utils/builtins.h"
+#include "utils/wait_event.h"
+
+PG_MODULE_MAGIC;
+
+PG_FUNCTION_INFO_V1(test_large_files_offset_size);
+Datum
+test_large_files_offset_size(PG_FUNCTION_ARGS)
+{
+	PG_RETURN_INT32(sizeof(pgoff_t));
+}
+
+PG_FUNCTION_INFO_V1(test_large_files_test_4gb_boundary);
+Datum
+test_large_files_test_4gb_boundary(PG_FUNCTION_ARGS)
+{
+	File		file;
+	pgoff_t		large_offset = ((pgoff_t) 4294967296LL) + 1;
+	pgoff_t		expected_size = large_offset + 8;
+	pgoff_t		actual_size;
+	char		write_buf_0[8] = "OFFSET_0";
+	char		write_buf_large[8] = "TESTDATA";
+	char		read_buf[8];
+	int			nbytes;
+
+	file = OpenTemporaryFile(false);
+	if (file < 0)
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not create temporary file")));
+
+	nbytes = FileWrite(file, write_buf_0, 8, 0, WAIT_EVENT_DATA_FILE_WRITE);
+	if (nbytes != 8)
+	{
+		FileClose(file);
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not write at offset 0")));
+	}
+
+	nbytes = FileWrite(file, write_buf_large, 8, large_offset,
+					   WAIT_EVENT_DATA_FILE_WRITE);
+	if (nbytes != 8)
+	{
+		FileClose(file);
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not write at large offset")));
+	}
+
+	actual_size = FileSize(file);
+	if (actual_size < expected_size)
+	{
+		FileClose(file);
+		ereport(ERROR,
+				(errmsg("file size is %lld bytes, expected at least %lld bytes - offset truncated",
+						(long long) actual_size,
+						(long long) expected_size)));
+	}
+
+	memset(read_buf, 0, 8);
+	nbytes = FileRead(file, read_buf, 8, 0, WAIT_EVENT_DATA_FILE_READ);
+	if (nbytes != 8)
+	{
+		FileClose(file);
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not read from offset 0")));
+	}
+
+	if (memcmp(read_buf, write_buf_0, 8) != 0)
+	{
+		FileClose(file);
+		ereport(ERROR,
+				(errmsg("data at offset 0 was corrupted - write wrapped around")));
+	}
+
+	memset(read_buf, 0, 8);
+	nbytes = FileRead(file, read_buf, 8, large_offset,
+					  WAIT_EVENT_DATA_FILE_READ);
+	if (nbytes != 8)
+	{
+		FileClose(file);
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not read at large offset")));
+	}
+
+	if (memcmp(write_buf_large, read_buf, 8) != 0)
+	{
+		FileClose(file);
+		ereport(ERROR,
+				(errmsg("data mismatch at large offset")));
+	}
+
+	FileClose(file);
+
+	PG_RETURN_TEXT_P(cstring_to_text("4GB boundary test passed"));
+}
diff --git a/src/test/modules/test_large_files/test_large_files.control b/src/test/modules/test_large_files/test_large_files.control
new file mode 100644
index 0000000000..b0bff5bd86
--- /dev/null
+++ b/src/test/modules/test_large_files/test_large_files.control
@@ -0,0 +1,5 @@
+# test_large_files extension
+comment = 'Test module for large file I/O operations'
+default_version = '1.0'
+module_pathname = '$libdir/test_large_files'
+relocatable = true
-- 
2.46.0.windows.1