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.

doc/src/sgml/config.sgml

@@ -1442,12 +1442,12 @@ SET ENABLE_SEQSCAN TO OFF;
         </listitem>
         <listitem>
         <para>
-         <literal>fsync_writethrough</> (call <function>fsync()</> at each commit, forcing write-through of any disk write cache)
+         <literal>fsync</> (call <function>fsync()</> at each commit)
         </para>
         </listitem>
         <listitem>
         <para>
-         <literal>fsync</> (call <function>fsync()</> at each commit)
+         <literal>fsync_writethrough</> (call <function>fsync()</> at each commit, forcing write-through of any disk write cache)
         </para>
         </listitem>
         <listitem>
@@ -1457,12 +1457,11 @@ SET ENABLE_SEQSCAN TO OFF;
         </listitem>
        </itemizedlist>
        <para>
-        Not all of these choices are available on all platforms.
         The <literal>open_</>* options also use <literal>O_DIRECT</> if available.
+        Not all of these choices are available on all platforms.
         The default is the first method in the above list that is supported
-        by the platform.
-        The default is the first method in the above list that is supported
-        by the platform.  The default is not necessarily ideal; it might be
+        by the platform, except that <literal>fdatasync</> is the default on
+        Linux.  The default is not necessarily ideal; it might be
         necessary to change this setting or other aspects of your system
         configuration in order to create a crash-safe configuration or
         achieve optimal performance.

src/backend/storage/file/fd.c

@@ -256,12 +256,13 @@ static void RemovePgTempFilesInDir(const char *tmpdirname);
 int
 pg_fsync(int fd)
 {
-#ifndef HAVE_FSYNC_WRITETHROUGH_ONLY
-	if (sync_method != SYNC_METHOD_FSYNC_WRITETHROUGH)
-		return pg_fsync_no_writethrough(fd);
+	/* #if is to skip the sync_method test if there's no need for it */
+#if defined(HAVE_FSYNC_WRITETHROUGH) && !defined(FSYNC_WRITETHROUGH_IS_FSYNC)
+	if (sync_method == SYNC_METHOD_FSYNC_WRITETHROUGH)
+		return pg_fsync_writethrough(fd);
 	else
 #endif
-		return pg_fsync_writethrough(fd);
+		return pg_fsync_no_writethrough(fd);
 }
 
 

src/backend/utils/misc/postgresql.conf.sample

@@ -153,7 +153,7 @@
 #wal_sync_method = fsync		# the default is the first option 
 					# supported by the operating system:
 					#   open_datasync
-					#   fdatasync
+					#   fdatasync (default on Linux)
 					#   fsync
 					#   fsync_writethrough
 					#   open_sync

src/include/access/xlogdefs.h

@@ -110,12 +110,12 @@ typedef uint32 TimeLineID;
 #endif
 #endif
 
-#if defined(OPEN_DATASYNC_FLAG)
+#if defined(PLATFORM_DEFAULT_SYNC_METHOD)
+#define DEFAULT_SYNC_METHOD		PLATFORM_DEFAULT_SYNC_METHOD
+#elif defined(OPEN_DATASYNC_FLAG)
 #define DEFAULT_SYNC_METHOD		SYNC_METHOD_OPEN_DSYNC
 #elif defined(HAVE_FDATASYNC)
 #define DEFAULT_SYNC_METHOD		SYNC_METHOD_FDATASYNC
-#elif defined(HAVE_FSYNC_WRITETHROUGH_ONLY)
-#define DEFAULT_SYNC_METHOD		SYNC_METHOD_FSYNC_WRITETHROUGH
 #else
 #define DEFAULT_SYNC_METHOD		SYNC_METHOD_FSYNC
 #endif

src/include/port/linux.h

@@ -12,3 +12,11 @@
  * to have a kernel version test here.
  */
 #define HAVE_LINUX_EIDRM_BUG
+
+/*
+ * Set the default wal_sync_method to fdatasync.  With recent Linux versions,
+ * xlogdefs.h's normal rules will prefer open_datasync, which (a) doesn't
+ * perform better and (b) causes outright failures on ext4 data=journal
+ * filesystems, because those don't support O_DIRECT.
+ */
+#define PLATFORM_DEFAULT_SYNC_METHOD	SYNC_METHOD_FDATASYNC

src/include/port/win32.h

@@ -34,15 +34,19 @@
 /* Must be here to avoid conflicting with prototype in windows.h */
 #define mkdir(a,b)	mkdir(a)
 
-#define HAVE_FSYNC_WRITETHROUGH
-#define HAVE_FSYNC_WRITETHROUGH_ONLY
 #define ftruncate(a,b)	chsize(a,b)
-/*
- *	Even though we don't support 'fsync' as a wal_sync_method,
- *	we do fsync() a few other places where _commit() is just fine.
- */
+
+/* Windows doesn't have fsync() as such, use _commit() */
 #define fsync(fd) _commit(fd)
 
+/*
+ * For historical reasons, we allow setting wal_sync_method to
+ * fsync_writethrough on Windows, even though it's really identical to fsync
+ * (both code paths wind up at _commit()).
+ */
+#define HAVE_FSYNC_WRITETHROUGH
+#define FSYNC_WRITETHROUGH_IS_FSYNC
+
 #define USES_WINSOCK
 
 /* defines for dynamic linking on Win32 platform */