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Reword fsync and full_page_writes docs to be clearer about when to turn

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them off.

Josh Berkus, with slight wording changes by me.
This commit is contained in:
Bruce Momjian 2010-05-31 15:50:48 +00:00
parent e0b581acd2
commit 6f1932c249
1 changed files with 19 additions and 32 deletions
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51
doc/src/sgml/config.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.279 2010/05/26 23:49:18 tgl Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.280 2010/05/31 15:50:48 momjian Exp $ -->
<chapter Id="runtime-config">
<title>Server Configuration</title>
@ -1413,34 +1413,23 @@ SET ENABLE_SEQSCAN TO OFF;
</para>
<para>
However, using <varname>fsync</varname> results in a
performance penalty: when a transaction is committed,
<productname>PostgreSQL</productname> must wait for the
operating system to flush the write-ahead log to disk. When
<varname>fsync</varname> is disabled, the operating system is
allowed to do its best in buffering, ordering, and delaying
writes. This can result in significantly improved performance.
However, if the system crashes, the results of the last few
committed transactions might be completely lost, or worse,
might appear partially committed, leaving the database in an
inconsistent state. In the
worst case, unrecoverable data corruption might occur.
(Crashes of the database software itself are <emphasis>not</>
a risk factor here. Only an operating-system-level crash
creates a risk of corruption.)
While turning off <varname>fsync</varname> is often a performance
benefit, this can result in unrecoverable data corruption in
the event of an unexpected system shutdown or crash. Thus it
is only advisable to turn off <varname>fsync</varname> if
you can easily recreate your entire database from external
data.
</para>
<para>
Due to the risks involved, there is no universally correct
setting for <varname>fsync</varname>. Some administrators
always disable <varname>fsync</varname>, while others only
turn it off during initial bulk data loads, where there is a clear
restart point if something goes wrong. Others
always leave <varname>fsync</varname> enabled. The default is
to enable <varname>fsync</varname>, for maximum reliability.
If you trust your operating system, your hardware, and your
utility company (or your battery backup), you can consider
disabling <varname>fsync</varname>.
Examples of safe circumstances for turning off
<varname>fsync</varname> include the initial loading a new
database cluster from a backup file, using a database cluster
for processing statistics on an hourly basis which is then
recreated, or for a reporting read-only database clone which
gets recreated frequently and is not used for failover. High
quality hardware alone is not a sufficient justification for
turning off <varname>fsync</varname>.
</para>
<para>
@ -1572,12 +1561,10 @@ SET ENABLE_SEQSCAN TO OFF;
<para>
Turning this parameter off speeds normal operation, but
might lead to a corrupt database after an operating system crash
or power failure. The risks are similar to turning off
<varname>fsync</>, though smaller. It might be safe to turn off
this parameter if you have hardware (such as a battery-backed disk
controller) or file-system software that reduces
the risk of partial page writes to an acceptably low level (e.g., ZFS).
might lead to either unrecoverable data corruption, or silent
data corruption, after a system failure. The risks are similar to turning off
<varname>fsync</varname>, though smaller, and it should be turned off
only based on the same circumstances recommended for that parameter.
</para>
<para>