Re: [HACKERS] no universally correct setting for fsync

On 5/7/10 5:13 PM, Tom Lane wrote:
> Josh Berkus <josh(at)agliodbs(dot)com> writes:
>> This is what I have to replace the current fsync entry in config.sgml.
>
> s/unexpected shutdown/system crash/, perhaps. The wording you have
> suggests that a forced Postgres stoppage produces a problem, which it
> doesn't. It takes a failure at the OS level or below to cause a
> problem.

I actually meant "unexpected *system* shutdown", i.e. power-out. A lot
of people think "crash" just means kernel dump, whereas a UPS failure or
tripped power cord is a lot more likely (except maybe on Windows).

Revised:

==================

<varlistentry id="guc-fsync" xreflabel="fsync">
<indexterm>
<primary><varname>fsync</> configuration parameter</primary>
</indexterm>
<term><varname>fsync</varname> (<type>boolean</type>)</term>
<listitem>
<para>
If this parameter is on, the <productname>PostgreSQL</> server
will try to make sure that updates are physically written to
disk, by issuing <function>fsync()</> system calls or various
equivalent methods (see <xref linkend="guc-wal-sync-method">).
This ensures that the database cluster can recover to a
consistent state after an operating system or hardware crash.
</para>

<para>
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>
In many situations, turning off <xref
linkend="guc-synchronous-commit">
for noncritical transactions can provide much of the potential
performance benefit of turning off <varname>fsync</varname>, without
the attendant risks of data corruption.
</para>

<para>
<varname>fsync</varname> can only be set in the
<filename>postgresql.conf</>
file or on the server command line.
If you turn this parameter off, also consider turning off
<xref linkend="guc-full-page-writes">.
</para>
</listitem>
</varlistentry>

--
-- Josh Berkus
PostgreSQL Experts Inc.
http://www.pgexperts.com