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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 7887db612c
commit c1db529642
2 changed files with 13 additions and 4 deletions
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6
doc/src/sgml/config.sgml
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@ -1570,7 +1570,11 @@ SET ENABLE_SEQSCAN TO OFF;
<para>
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.
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 utility <filename>src/tools/fsync</> in the PostgreSQL source tree
can do performance testing of various fsync methods.

11
doc/src/sgml/wal.sgml
View File

@ -85,7 +85,9 @@
by unchecking <literal>My Computer\Open\{select disk
drive}\Properties\Hardware\Properties\Policies\Enable write caching on
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>
@ -529,8 +531,10 @@
The <xref linkend="guc-wal-sync-method"> parameter determines how
<productname>PostgreSQL</productname> will ask the kernel to force
<acronym>WAL</acronym> updates out to disk.
All the options should be the same in terms of reliability,
but it's quite platform-specific which one will be the fastest.
With the exception of <literal>fsync_writethrough</>, which can sometimes
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>
has been turned off.
</para>
@ -590,6 +594,7 @@
irrecoverable data corruption. Administrators should try to ensure
that disks holding <productname>PostgreSQL</productname>'s
<acronym>WAL</acronym> log files do not make such false reports.
(See <xref linkend="wal-reliability">.)
</para>
<para>