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.

5 files changed, 26 insertions, 13 deletions

diff --git a/src/backend/storage/file/fd.c b/src/backend/storage/file/fd.c
index 89a2d4ae24a..613834eb96d 100644
--- a/src/backend/storage/file/fd.c
+++ b/src/backend/storage/file/fd.c
@@ -257,12 +257,13 @@ 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);
 }
 
 
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index e1f5ab66ed5..a3880da7dac 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -157,7 +157,7 @@
 #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
diff --git a/src/include/access/xlogdefs.h b/src/include/access/xlogdefs.h
index 0760b259308..1da1ae0db0c 100644
--- a/src/include/access/xlogdefs.h
+++ b/src/include/access/xlogdefs.h
@@ -123,12 +123,12 @@ 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
diff --git a/src/include/port/linux.h b/src/include/port/linux.h
index 0f4432a4eff..e2a4c4a114d 100644
--- a/src/include/port/linux.h
+++ b/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
diff --git a/src/include/port/win32.h b/src/include/port/win32.h
index 7338b115c46..1473d9e8c3c 100644
--- a/src/include/port/win32.h
+++ b/src/include/port/win32.h
@@ -34,14 +34,18 @@
 /* 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)
+
+/* Windows doesn't have fsync() as such, use _commit() */
+#define fsync(fd) _commit(fd)
+
 /*
- *	Even though we don't support 'fsync' as a wal_sync_method,
- *	we do fsync() a few other places where _commit() is just fine.
+ * 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 fsync(fd) _commit(fd)
+#define HAVE_FSYNC_WRITETHROUGH
+#define FSYNC_WRITETHROUGH_IS_FSYNC
 
 #define USES_WINSOCK