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Force default wal_sync_method to be fdatasync on Linux.

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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.
This commit is contained in:
Tom Lane
2010-12-08 20:01:29 -05:00
parent 5271c3cd80
commit 8b77981f89
6 changed files with 36 additions and 20 deletions
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9
doc/src/sgml/config.sgml
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@ -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>