8000 Force default wal_sync_method to be fdatasync on Linux. · patchsoft/postgres@6b2c0ea · GitHub
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Force default wal_sync_method to be fdatasync on Linux.
Recent versions of the Linux system header files cause xlogdefs.h to believe that open_datasync should be the default sync method, whereas formerly fdatasync was the default on Linux. open_datasync is a bad choice, first because it doesn't actually outperform fdatasync (in fact the reverse), and second because we try to use O_DIRECT with it, causing failures on certain filesystems (e.g., ext4 with data=journal option). This part of the patch is largely per a proposal from Marti Raudsepp. More extensive changes are likely to follow in HEAD, but this is as much change as we want to back-patch. Also clean up confusing code and incorrect documentation surrounding the fsync_writethrough option. Those changes shouldn't result in any actual behavioral change, but I chose to back-patch them anyway to keep the branches looking similar in this area. In 9.0 and HEAD, also do some copy-editing on the WAL Reliability documentation section. Back-patch to all supported branches, since any of them might get used on modern Linux versions.
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-18
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6 files changed

+35
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lines changed

‎doc/src/sgml/config.sgml

Lines changed: 5 additions & 3 deletions
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@@ -1347,12 +1347,12 @@ SET ENABLE_SEQSCAN TO OFF;
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</listitem>
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<listitem>
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<para>
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<literal>fsync_writethrough</> (call <function>fsync()</> at each commit, forcing write-through of any disk write cache)
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<literal>fsync</> (call <function>fsync()</> at each commit)
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</para>
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</listitem>
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<listitem>
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<para>
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<literal>fsync</> (call <function>fsync()</> at each commit)
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<literal>fsync_writethrough</> (call <function>fsync()</> at each commit, forcing write-through of any disk write cache)
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</para>
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</listitem>
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<listitem>
@@ -1363,7 +1363,9 @@ SET ENABLE_SEQSCAN TO OFF;
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</itemizedlist>
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<para>
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Not all of these choices are available on all platforms.
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The default is the first method in the above list that is supported.
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The default is the first method in the above list that is supported
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by the platform, except that <literal>fdatasync</> is the default on
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Linux.
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This option can be set at server start or in the
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<filename>postgresql.conf</filename> file.
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</para>

‎src/backend/access/transam/xlog.c

Lines changed: 5 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -92,18 +92,18 @@
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#endif
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#endif
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#if defined(OPEN_DATASYNC_FLAG)
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#if defined(PLATFORM_DEFAULT_SYNC_METHOD)
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#define DEFAULT_SYNC_METHOD_STR PLATFORM_DEFAULT_SYNC_METHOD_STR
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#define DEFAULT_SYNC_METHOD PLATFORM_DEFAULT_SYNC_METHOD
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#define DEFAULT_SYNC_FLAGBIT PLATFORM_DEFAULT_SYNC_FLAGBIT
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#elif defined(OPEN_DATASYNC_FLAG)
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#define DEFAULT_SYNC_METHOD_STR "open_datasync"
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#define DEFAULT_SYNC_METHOD SYNC_METHOD_OPEN
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#define DEFAULT_SYNC_FLAGBIT OPEN_DATASYNC_FLAG
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#elif defined(HAVE_FDATASYNC)
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#define DEFAULT_SYNC_METHOD_STR "fdatasync"
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#define DEFAULT_SYNC_METHOD SYNC_METHOD_FDATASYNC
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#define DEFAULT_SYNC_FLAGBIT 0
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#elif defined(HAVE_FSYNC_WRITETHROUGH_ONLY)
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#define DEFAULT_SYNC_METHOD_STR "fsync_writethrough"
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#define DEFAULT_SYNC_METHOD SYNC_METHOD_FSYNC_WRITETHROUGH
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#define DEFAULT_SYNC_FLAGBIT 0
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#else
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#define DEFAULT_SYNC_METHOD_STR "fsync"
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#define DEFAULT_SYNC_METHOD SYNC_METHOD_FSYNC

‎src/backend/storage/file/fd.c

Lines changed: 5 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -237,12 +237,13 @@ static void RemovePgTempFilesInDir(const char *tmpdirname);
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int
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pg_fsync(int fd)
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{
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#ifndef HAVE_FSYNC_WRITETHROUGH_ONLY
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if (sync_method != SYNC_METHOD_FSYNC_WRITETHROUGH)
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return pg_fsync_no_writethrough(fd);
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/* #if is to skip the sync_method test if there's no need for it */
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#if defined(HAVE_FSYNC_WRITETHROUGH) && !defined(FSYNC_WRITETHROUGH_IS_FSYNC)
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if (sync_method == SYNC_METHOD_FSYNC_WRITETHROUGH)
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return pg_fsync_writethrough(fd);
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else
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#endif
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return pg_fsync_writethrough(fd);
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return pg_fsync_no_writethrough(fd);
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}
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‎src/backend/utils/misc/postgresql.conf.sample

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -138,7 +138,7 @@
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#wal_sync_method = fsync # the default is the first option
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# supported by the operating system:
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# open_datasync
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# fdatasync
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# fdatasync (default on Linux)
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# fsync
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# fsync_writethrough
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# open_sync

‎src/include/port/linux.h

Lines changed: 10 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -10,3 +10,13 @@
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* to have a kernel version test here.
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*/
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#define HAVE_LINUX_EIDRM_BUG
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/*
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* Set the default wal_sync_method to fdatasync. With recent Linux versions,
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* xlogdefs.h's normal rules will prefer open_datasync, which (a) doesn't
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* perform better and (b) causes outright failures on ext4 data=journal
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* filesystems, because those don't support O_DIRECT.
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*/
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#define PLATFORM_DEFAULT_SYNC_METHOD_STR "fdatasync"
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#define PLATFORM_DEFAULT_SYNC_METHOD SYNC_METHOD_FDATASYNC
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#define PLATFORM_DEFAULT_SYNC_FLAGBIT 0

‎src/include/port/win32.h

Lines changed: 9 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -16,14 +16,18 @@
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#define mkdir(a,b) mkdir(a)
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#define HAVE_FSYNC_WRITETHROUGH
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#define HAVE_FSYNC_WRITETHROUGH_ONLY
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#define ftruncate(a,b) chsize(a,b)
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/* Windows doesn't have fsync() as such, use _commit() */
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#define fsync(fd) _commit(fd)
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/*
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* Even though we don't support 'fsync' as a wal_sync_method,
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* we do fsync() a few other places where _commit() is just fine.
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* For historical reasons, we allow setting wal_sync_method to
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* fsync_writethrough on Windows, even though it's really identical to fsync
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* (both code paths wind up at _commit()).
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*/
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#define fsync(fd) _commit(fd)
29+
#define HAVE_FSYNC_WRITETHROUGH
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#define FSYNC_WRITETHROUGH_IS_FSYNC
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#define USES_WINSOCK
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