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.

1 files changed, 5 insertions, 6 deletions

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 3aa74870f88..f3287219627 100644
--- a/doc/src/sgml/config.sgml
+++ b/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.