Re: [HACKERS] New wal_sync_method for Darwin?

Applied.

---------------------------------------------------------------------------

Bruce Momjian wrote:

Attached is an updated version of your Darwin Fsync-writethrough patch.
I did much more restructuring because we do fsync in more places than
just WAL (like at checkpoints), and we should follow the WAL
write-through in those places too if that method is used for fsync of
WAL, rather than the normal fsync without the force-writethrough.

I restructured the code so now pg_fsync does writethrough or not based
on the wal setting. My feeling is you need WAL writethrough, you need
it for other fsyncs as well. Write-through was added in an 8.0.X
subrelease, and because it was a subrelease, the patch did a minimal
amount of code change. This change is the restructuring that makes
sense for 8.1.

---------------------------------------------------------------------------

Chris Campbell wrote:

Thanks, Bruce! Here's the patch (would you rather patches be attached
files, or inline?).

Some explanation. In the current code, there is no "fsync" on Windows,
only "fsync_writethrough". But, internally, it behaves identically to
"fsync". The only special case was for the GUC strings.

The patch changes it thusly: since "fsync" and "fsync_writethrough"
cannot be identical on Darwin, I added a new constant for
SYNC_METHOD_FSYNC_WRITE_THROUGH (3). A GUC value of
"fsync_writethrough" sets fsync_method to this new constant.

On Windows, the fsync_writethrough case falls through to the fsync case
(which simply runs fsync). On Darwin, the fsync_writethrough case
executes fcntl(openLogFile, F_FULLFSYNC, 0), which does everything that
fsync does, and then flushes the disk's write cache. The code does not
need to call fsync directly, just fcntl.

The performance of a script that executes 11,000 INSERT statements
(outside a transaction) is pretty slow, since it requires flushing the
disk write cache after EVERY query (transaction commit). I waited about
15 minutes before I just killed it. But if you do all the inserts
inside a transaction, then I only pay the cache flush price once at
transaction commit, and it's quite usable.

I asked around on IRC, and the only place where flushing the disk cache
is important is when syncing a WAL file. So I didn't change the
behavior of fsync() globally for the code...just for WAL syncs.

I added a new #define called HAVE_FSYNC_WRITE_THROUGH, which is defined
only by the win32.h and darwin.h port headers. This controls the
availability of the "fsync_writethrough" GUC setting. Thanks to the
port headers, no configure test is needed.

Thanks! Let me know what you think of the patch, and tell me what else
I can do.

- Chris

-- 
Bruce Momjian                        |  http://candle.pha.pa.us
pgman@candle.pha.pa.us               |  (610) 359-1001
+  If your life is a hard drive,     |  13 Roberts Road
+  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073

Index: doc/src/sgml/runtime.sgml
===================================================================
RCS file: /cvsroot/pgsql/doc/src/sgml/runtime.sgml,v
retrieving revision 1.319
diff -c -c -r1.319 runtime.sgml
*** doc/src/sgml/runtime.sgml	15 May 2005 00:26:18 -0000	1.319
--- doc/src/sgml/runtime.sgml	17 May 2005 22:03:49 -0000
***************
*** 1595,1601 ****
values are
<literal>fsync</> (call <function>fsync()</> at each commit),
<literal>fdatasync</> (call <function>fdatasync()</> at each commit),
!         <literal>fsync_writethrough</> (call <function>_commit()</> at each commit on Windows),
<literal>open_sync</> (write WAL files with <function>open()</> option <symbol>O_SYNC</>), and
<literal>open_datasync</> (write WAL files with <function>open()</> option <symbol>O_DSYNC</>).
Not all of these choices are available on all platforms.
--- 1595,1601 ----
values are
<literal>fsync</> (call <function>fsync()</> at each commit),
<literal>fdatasync</> (call <function>fdatasync()</> at each commit),
!         <literal>fsync_writethrough</> (force write-through of any disk write cache),
<literal>open_sync</> (write WAL files with <function>open()</> option <symbol>O_SYNC</>), and
<literal>open_datasync</> (write WAL files with <function>open()</> option <symbol>O_DSYNC</>).
Not all of these choices are available on all platforms.
Index: src/backend/access/transam/xlog.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/access/transam/xlog.c,v
retrieving revision 1.191
diff -c -c -r1.191 xlog.c
*** src/backend/access/transam/xlog.c	10 May 2005 22:27:29 -0000	1.191
--- src/backend/access/transam/xlog.c	17 May 2005 22:03:53 -0000
***************
*** 51,61 ****
* default method.	We assume that fsync() is always available, and that
* configure determined whether fdatasync() is.
*/
- #define SYNC_METHOD_FSYNC		0
- #define SYNC_METHOD_FDATASYNC	1
- #define SYNC_METHOD_OPEN		2		/* used for both O_SYNC and
- 										 * O_DSYNC */
- 
#if defined(O_SYNC)
#define OPEN_SYNC_FLAG	   O_SYNC
#else
--- 51,56 ----
***************
*** 79,99 ****
#define DEFAULT_SYNC_METHOD_STR    "open_datasync"
#define DEFAULT_SYNC_METHOD		   SYNC_METHOD_OPEN
#define DEFAULT_SYNC_FLAGBIT	   OPEN_DATASYNC_FLAG
! #else
! #if defined(HAVE_FDATASYNC)
#define DEFAULT_SYNC_METHOD_STR   "fdatasync"
#define DEFAULT_SYNC_METHOD		  SYNC_METHOD_FDATASYNC
#define DEFAULT_SYNC_FLAGBIT	  0
! #else
! #ifndef FSYNC_IS_WRITE_THROUGH
#define DEFAULT_SYNC_METHOD_STR   "fsync"
#else
#define DEFAULT_SYNC_METHOD_STR   "fsync_writethrough"
! #endif
! #define DEFAULT_SYNC_METHOD		  SYNC_METHOD_FSYNC
#define DEFAULT_SYNC_FLAGBIT	  0
#endif
- #endif

/* User-settable parameters */
--- 74,92 ----
#define DEFAULT_SYNC_METHOD_STR    "open_datasync"
#define DEFAULT_SYNC_METHOD		   SYNC_METHOD_OPEN
#define DEFAULT_SYNC_FLAGBIT	   OPEN_DATASYNC_FLAG
! #elif defined(HAVE_FDATASYNC)
#define DEFAULT_SYNC_METHOD_STR   "fdatasync"
#define DEFAULT_SYNC_METHOD		  SYNC_METHOD_FDATASYNC
#define DEFAULT_SYNC_FLAGBIT	  0
! #elif !defined(HAVE_FSYNC_WRITETHROUGH_ONLY)
#define DEFAULT_SYNC_METHOD_STR   "fsync"
+ #define DEFAULT_SYNC_METHOD		  SYNC_METHOD_FSYNC
+ #define DEFAULT_SYNC_FLAGBIT	  0
#else
#define DEFAULT_SYNC_METHOD_STR   "fsync_writethrough"
! #define DEFAULT_SYNC_METHOD		  SYNC_METHOD_FSYNC_WRITETHROUGH
#define DEFAULT_SYNC_FLAGBIT	  0
#endif

/* User-settable parameters */
***************
*** 122,128 ****

/* these are derived from XLOG_sync_method by assign_xlog_sync_method */
! static int sync_method = DEFAULT_SYNC_METHOD;
static int open_sync_bit = DEFAULT_SYNC_FLAGBIT;

#define XLOG_SYNC_BIT  (enableFsync ? open_sync_bit : 0)
--- 115,121 ----

/* these are derived from XLOG_sync_method by assign_xlog_sync_method */
! int sync_method = DEFAULT_SYNC_METHOD;
static int open_sync_bit = DEFAULT_SYNC_FLAGBIT;

#define XLOG_SYNC_BIT (enableFsync ? open_sync_bit : 0)
***************
*** 5249,5264 ****
int new_sync_method;
int new_sync_bit;

- #ifndef FSYNC_IS_WRITE_THROUGH
if (pg_strcasecmp(method, "fsync") == 0)
- #else
- 	/* Win32 fsync() == _commit(), which writes through a write cache */
- 	if (pg_strcasecmp(method, "fsync_writethrough") == 0)
- #endif
{
new_sync_method = SYNC_METHOD_FSYNC;
new_sync_bit = 0;
}
#ifdef HAVE_FDATASYNC
else if (pg_strcasecmp(method, "fdatasync") == 0)
{
--- 5242,5259 ----
int			new_sync_method;
int			new_sync_bit;

if (pg_strcasecmp(method, "fsync") == 0)
{
new_sync_method = SYNC_METHOD_FSYNC;
new_sync_bit = 0;
}
+ #ifdef HAVE_FSYNC_WRITETHROUGH
+ 	else if (pg_strcasecmp(method, "fsync_writethrough") == 0)
+ 	{
+ 		new_sync_method = SYNC_METHOD_FSYNC_WRITETHROUGH;
+ 		new_sync_bit = 0;
+ 	}
+ #endif
#ifdef HAVE_FDATASYNC
else if (pg_strcasecmp(method, "fdatasync") == 0)
{
***************
*** 5328,5339 ****
switch (sync_method)
{
case SYNC_METHOD_FSYNC:
! 			if (pg_fsync(openLogFile) != 0)
ereport(PANIC,
(errcode_for_file_access(),
errmsg("could not fsync log file %u, segment %u: %m",
openLogId, openLogSeg)));
break;
#ifdef HAVE_FDATASYNC
case SYNC_METHOD_FDATASYNC:
if (pg_fdatasync(openLogFile) != 0)
--- 5323,5343 ----
switch (sync_method)
{
case SYNC_METHOD_FSYNC:
! 			if (pg_fsync_no_writethrough(openLogFile) != 0)
ereport(PANIC,
(errcode_for_file_access(),
errmsg("could not fsync log file %u, segment %u: %m",
openLogId, openLogSeg)));
break;
+ #ifdef HAVE_FSYNC_WRITETHROUGH
+ 		case SYNC_METHOD_FSYNC_WRITETHROUGH:
+ 			if (pg_fsync_writethrough(openLogFile) != 0)
+ 				ereport(PANIC,
+ 						(errcode_for_file_access(),
+ 					errmsg("could not fsync write-through log file %u, segment %u: %m",
+ 						   openLogId, openLogSeg)));
+ 			break;
+ #endif
#ifdef HAVE_FDATASYNC
case SYNC_METHOD_FDATASYNC:
if (pg_fdatasync(openLogFile) != 0)
Index: src/backend/storage/file/fd.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/storage/file/fd.c,v
retrieving revision 1.115
diff -c -c -r1.115 fd.c
*** src/backend/storage/file/fd.c	31 Dec 2004 22:00:51 -0000	1.115
--- src/backend/storage/file/fd.c	17 May 2005 22:03:54 -0000
***************
*** 232,242 ****

/*
!  * pg_fsync --- same as fsync except does nothing if enableFsync is off
*/
int
pg_fsync(int fd)
{
if (enableFsync)
return fsync(fd);
else
--- 232,258 ----

/*
!  * pg_fsync --- do fsync with or without writethrough
*/
int
pg_fsync(int fd)
{
+ #ifndef HAVE_FSYNC_WRITETHROUGH_ONLY
+ 	if (sync_method != SYNC_METHOD_FSYNC_WRITETHROUGH)
+ 		return pg_fsync_no_writethrough(fd);
+ 	else
+ #endif
+ 		return pg_fsync_writethrough(fd);
+ }
+ 
+ 
+ /*
+  * pg_fsync_no_writethrough --- same as fsync except does nothing if
+  *	enableFsync is off
+  */
+ int
+ pg_fsync_no_writethrough(int fd)
+ {
if (enableFsync)
return fsync(fd);
else
***************
*** 244,249 ****
--- 260,283 ----
}

/*
+  * pg_fsync_writethrough
+  */
+ int
+ pg_fsync_writethrough(int fd)
+ {
+ 	if (enableFsync)
+ #ifdef WIN32
+ 		return _commit(fd);
+ #elif defined(__darwin__)
+ 		return (fcntl(fd, F_FULLFSYNC, 0) == -1) ? -1 : 0;
+ #else
+ 		return -1;
+ #endif
+ 	else
+ 		return 0;
+ }
+ 
+ /*
* pg_fdatasync --- same as fdatasync except does nothing if enableFsync is off
*
* Not all platforms have fdatasync; treat as fsync if not available.
Index: src/include/access/xlog.h
===================================================================
RCS file: /cvsroot/pgsql/src/include/access/xlog.h,v
retrieving revision 1.60
diff -c -c -r1.60 xlog.h
*** src/include/access/xlog.h	28 Apr 2005 21:47:17 -0000	1.60
--- src/include/access/xlog.h	17 May 2005 22:03:56 -0000
***************
*** 75,80 ****
--- 75,87 ----
*/
#define XLOG_NO_TRAN			XLR_INFO_MASK

+ /* Sync methods */
+ #define SYNC_METHOD_FSYNC		0
+ #define SYNC_METHOD_FDATASYNC	1
+ #define SYNC_METHOD_OPEN		2			/* for O_SYNC and O_DSYNC */
+ #define SYNC_METHOD_FSYNC_WRITETHROUGH	3
+ extern int	sync_method;
+ 
/*
* List of these structs is used to pass data to XLogInsert().
*
Index: src/include/port/darwin.h
===================================================================
RCS file: /cvsroot/pgsql/src/include/port/darwin.h,v
retrieving revision 1.6
diff -c -c -r1.6 darwin.h
*** src/include/port/darwin.h	23 Dec 2003 03:31:30 -0000	1.6
--- src/include/port/darwin.h	17 May 2005 22:03:57 -0000
***************
*** 1 ****
--- 1,4 ----
#define __darwin__	1
+ 
+ #define HAVE_FSYNC_WRITETHROUGH
+ 
Index: src/include/port/win32.h
===================================================================
RCS file: /cvsroot/pgsql/src/include/port/win32.h,v
retrieving revision 1.44
diff -c -c -r1.44 win32.h
*** src/include/port/win32.h	24 Mar 2005 04:36:19 -0000	1.44
--- src/include/port/win32.h	17 May 2005 22:03:57 -0000
***************
*** 16,23 ****
#define mkdir(a,b)	mkdir(a)

! #define fsync(a) _commit(a)
! #define FSYNC_IS_WRITE_THROUGH
#define ftruncate(a,b) chsize(a,b)

#define USES_WINSOCK
--- 16,23 ----
#define mkdir(a,b)	mkdir(a)

! #define HAVE_FSYNC_WRITETHROUGH
! #define HAVE_FSYNC_WRITETHROUGH_ONLY
#define ftruncate(a,b) chsize(a,b)

#define USES_WINSOCK
Index: src/include/storage/fd.h
===================================================================
RCS file: /cvsroot/pgsql/src/include/storage/fd.h,v
retrieving revision 1.50
diff -c -c -r1.50 fd.h
*** src/include/storage/fd.h	31 Dec 2004 22:03:42 -0000	1.50
--- src/include/storage/fd.h	17 May 2005 22:03:57 -0000
***************
*** 89,94 ****
--- 89,96 ----
SubTransactionId parentSubid);
extern void RemovePgTempFiles(void);
extern int	pg_fsync(int fd);
+ extern int	pg_fsync_no_writethrough(int fd);
+ extern int	pg_fsync_writethrough(int fd);
extern int	pg_fdatasync(int fd);

/* Filename components for OpenTemporaryFile */

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-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 359-1001
  +  If your life is a hard drive,     |  13 Roberts Road
  +  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073