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.

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 15645c31d09e6c69bbffa1a597e82fce3c76895f..ffc0d9cc14200227b90972d4e853e38bd58a1a86 100644 (file)
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -1404,12 +1404,12 @@ 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>
@@ -1419,10 +1419,11 @@ SET ENABLE_SEQSCAN TO OFF;
         </listitem>
        </itemizedlist>
        <para>
+        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 <literal>open_</>* options also use <literal>O_DIRECT</> if available.
+        by the platform, except that <literal>fdatasync</> is the default on
+        Linux.
         This parameter can only be set in the <filename>postgresql.conf</>
         file or on the server command line.
        </para>

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 024093c51fe06c85298699d89a1ab1fd4ed96e26..594437badc0f41e7bd2826bea88007587ebc4be6 100644 (file)
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -93,7 +93,11 @@
 #endif
 #endif
 
-#if defined(OPEN_DATASYNC_FLAG)
+#if defined(PLATFORM_DEFAULT_SYNC_METHOD)
+#define DEFAULT_SYNC_METHOD_STR        PLATFORM_DEFAULT_SYNC_METHOD_STR
+#define DEFAULT_SYNC_METHOD            PLATFORM_DEFAULT_SYNC_METHOD
+#define DEFAULT_SYNC_FLAGBIT   PLATFORM_DEFAULT_SYNC_FLAGBIT
+#elif defined(OPEN_DATASYNC_FLAG)
 #define DEFAULT_SYNC_METHOD_STR "open_datasync"
 #define DEFAULT_SYNC_METHOD            SYNC_METHOD_OPEN
 #define DEFAULT_SYNC_FLAGBIT   OPEN_DATASYNC_FLAG
@@ -101,10 +105,6 @@
 #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_writethrough"
-#define DEFAULT_SYNC_METHOD            SYNC_METHOD_FSYNC_WRITETHROUGH
-#define DEFAULT_SYNC_FLAGBIT   0
 #else
 #define DEFAULT_SYNC_METHOD_STR "fsync"
 #define DEFAULT_SYNC_METHOD            SYNC_METHOD_FSYNC

diff --git a/src/backend/storage/file/fd.c b/src/backend/storage/file/fd.c
index 127d7b605da0f7cca8a1734dfa963fcf92d77fd9..052007dca650179274d048469890cbe47f4a09e5 100644 (file)
--- a/src/backend/storage/file/fd.c
+++ b/src/backend/storage/file/fd.c
@@ -237,12 +237,13 @@ 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 115b3765ffd16c680c4723326a407f2ac401c812..83c5b8678fbb773c1c01578f2777cc58fccb7614 100644 (file)
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/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

diff --git a/src/include/port/linux.h b/src/include/port/linux.h
index 191338c99fda54af9c9554751f7768645e76a1f2..30e178db554e1b162b7978c7214fa3cb7cff2bf9 100644 (file)
--- a/src/include/port/linux.h
+++ b/src/include/port/linux.h
@@ -12,3 +12,13 @@
  * 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_STR "fdatasync"
+#define PLATFORM_DEFAULT_SYNC_METHOD   SYNC_METHOD_FDATASYNC
+#define PLATFORM_DEFAULT_SYNC_FLAGBIT  0

diff --git a/src/include/port/win32.h b/src/include/port/win32.h
index 50c711e121f9357d84e9efd63dc37448530375df..61c82e8d06cf4546de58e6b07c944c9a46187268 100644 (file)
--- a/src/include/port/win32.h
+++ b/src/include/port/win32.h
@@ -25,14 +25,18 @@
 /* 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