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Improve WAL reliability documentation, and add more cross-references to it.

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In particular, we are now more explicit about the fact that you may need
wal_sync_method=fsync_writethrough for crash-safety on some platforms,
including MaxOS X.  There's also now an explicit caution against assuming
that the default setting of wal_sync_method is either crash-safe or best
for performance.
This commit is contained in:
Robert Haas 2010-10-07 12:19:03 -04:00
parent 9e718e6116
commit 36f4b4e96b
2 changed files with 13 additions and 4 deletions
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6
doc/src/sgml/config.sgml
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@ -1459,7 +1459,11 @@ SET ENABLE_SEQSCAN TO OFF;
<para> <para>
Not all of these choices are available on all platforms. Not all of these choices are available on all platforms.
The default is the first method in the above list that is supported The default is the first method in the above list that is supported
by the platform. by the platform. The default is not necessarily best; it may be
necessary to change this setting, or other aspects of your system
configuration, in order to create a crash-safe configuration, as
discussed in <xref linkend="wal-reliability">, or to achieve best
performance.
The <literal>open_</>* options also use <literal>O_DIRECT</> if available. The <literal>open_</>* options also use <literal>O_DIRECT</> if available.
This parameter can only be set in the <filename>postgresql.conf</> This parameter can only be set in the <filename>postgresql.conf</>
file or on the server command line. file or on the server command line.

11
doc/src/sgml/wal.sgml
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@ -75,7 +75,9 @@
by unchecking <literal>My Computer\Open\{select disk by unchecking <literal>My Computer\Open\{select disk
drive}\Properties\Hardware\Properties\Policies\Enable write caching on drive}\Properties\Hardware\Properties\Policies\Enable write caching on
the disk</>. Also on Windows, <literal>fsync</> and the disk</>. Also on Windows, <literal>fsync</> and
<literal>fsync_writethrough</> never do write caching. <literal>fsync_writethrough</> never do write caching. The
<literal>fsync_writethrough</> option can also be used to disable
write caching on <productname>MacOS X</>.
</para> </para>
<para> <para>
@ -481,8 +483,10 @@
The <xref linkend="guc-wal-sync-method"> parameter determines how The <xref linkend="guc-wal-sync-method"> parameter determines how
<productname>PostgreSQL</productname> will ask the kernel to force <productname>PostgreSQL</productname> will ask the kernel to force
<acronym>WAL</acronym> updates out to disk. <acronym>WAL</acronym> updates out to disk.
All the options should be the same as far as reliability goes, With the exception of <literal>fsync_writethrough</>, which can sometimes
but it's quite platform-specific which one will be the fastest. force a flush of the disk cache even when other options do not do so,
all the options should be the same in terms of reliability.
However, it's quite platform-specific which one will be the fastest.
Note that this parameter is irrelevant if <varname>fsync</varname> Note that this parameter is irrelevant if <varname>fsync</varname>
has been turned off. has been turned off.
</para> </para>
@ -542,6 +546,7 @@
irrecoverable data corruption. Administrators should try to ensure irrecoverable data corruption. Administrators should try to ensure
that disks holding <productname>PostgreSQL</productname>'s that disks holding <productname>PostgreSQL</productname>'s
<acronym>WAL</acronym> log files do not make such false reports. <acronym>WAL</acronym> log files do not make such false reports.
(See <xref linkend="wal-reliability">.)
</para> </para>
<para> <para>