pg_fallocate

On Thu, Oct 31, 2013 at 9:16 AM, Mitsumasa KONDO
<kondo.mitsumasa@gmail.com> wrote:

I'l like to add fallocate() system call to improve sequential read/write
peformance. fallocate() system call is different from posix_fallocate() that
is zero-fille algorithm to reserve continues disk space. fallocate() is
almost less overhead alogotithm to reserve continues disk space than
posix_fallocate().

It will be needed by sorted checkpoint and more faster vacuum command in
near the future.

If you get more detail information, please see linux manual.

I go sight seeing in Dublin with Ishii-san now:-)

Our last attempts to improve performance in this area died in a fire
when it turned out that code that should have been an improvement fell
down over inexplicable ext4 behavior. I think, therefore, that
extensive benchmarking of this or any other proposed approach is
absolutely essential.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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On 10/31/13, 9:16 AM, Mitsumasa KONDO wrote:

I'l like to add fallocate() system call to improve sequential read/write
peformance. fallocate() system call is different from posix_fallocate()
that is zero-fille algorithm to reserve continues disk space.
fallocate() is almost less overhead alogotithm to reserve continues disk
space than posix_fallocate().

Your patch seems to be missing a bit that defines HAVE_FALLOCATE,
probably something in configure.in.

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On Thu, Oct 31, 2013 at 01:16:44PM +0000, Mitsumasa KONDO wrote:

--- a/src/backend/storage/file/fd.c
+++ b/src/backend/storage/file/fd.c
@@ -383,6 +383,21 @@ pg_flush_data(int fd, off_t offset, off_t amount)
return 0;
}

+/*
+ * pg_fallocate --- advise OS that the data pre-allocate continus file segments
+ * in physical disk.
+ *
+ * Not all platforms have fallocate. Some platforms only have posix_fallocate,
+ * but it ped zero fill to get pre-allocate file segmnets. It is not good
+ * peformance when extend new segmnets, so we don't use posix_fallocate.
+ */
+int
+pg_fallocate(File file, int flags, off_t offset, off_t nbytes)
+{
+#if defined(HAVE_FALLOCATE)
+	return fallocate(VfdCache[file].fd, flags, offset, nbytes);
+#endif
+}

You should set errno to ENOSYS and return -1 if HAVE_FALLOCATE isn't
defined.

--- a/src/backend/storage/smgr/md.c
+++ b/src/backend/storage/smgr/md.c
@@ -24,6 +24,7 @@
#include <unistd.h>
#include <fcntl.h>
#include <sys/file.h>
+#include <linux/falloc.h>

This would have to be wrapped in #ifdef HAVE_FALLOCATE or
HAVE_LINUX_FALLOC_H; if you want to create a wrapper around fallocate() you
should add PG defines for the flags, too. Otherwise it's probably easier to
just call fallocate() directly inside an #ifdef block as you did in xlog.c.

@@ -510,6 +511,10 @@ mdextend(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
* if bufmgr.c had to dump another buffer of the same file to make room
* for the new page's buffer.
*/
+
+	if(forknum == 1)
+		pg_fallocate(v->mdfd_vfd, FALLOC_FL_KEEP_SIZE, 0, RELSEG_SIZE);
+

Return value should be checked; if it's -1 and errno is something else than
ENOSYS or EOPNOTSUPP the disk space allocation failed and you must return an
error.

/ Oskari

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