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<chapter Id="runtime">

<listitem>

<para>

Method used for forcing WAL updates out to disk. Possible

values are

<literal>fsync</> (call <function>fsync()</> at each commit),

<literal>fdatasync</> (call <function>fdatasync()</> at each commit),

<literal>fsync_writethrough</> (force write-through of any disk write cache),

<literal>open_sync</> (write WAL files with <function>open()</> option <symbol>O_SYNC</>), and

<literal>open_datasync</> (write WAL files with <function>open()</> option <symbol>O_DSYNC</>).

values are:

</para>

<itemizedlist>

<listitem>

<para>

<literal>open_datasync</> (write WAL files with <function>open()</> option <symbol>O_DSYNC</>)

</para>

</listitem>

<listitem>

<para>

<literal>fdatasync</> (call <function>fdatasync()</> at each commit),

</para>

</listitem>

<listitem>

<para>

<literal>fsync</> (call <function>fsync()</> at each commit)

</para>

</listitem>

<listitem>

<para>

<literal>open_sync</> (write WAL files with <function>open()</> option <symbol>O_SYNC</>)

</para>

</listitem>

<listitem>

<para>

<literal>fsync_writethrough</> (force write-through of any disk write cache)

</para>

</listitem>

</itemizedlist>

<para>

Not all of these choices are available on all platforms.

The top-most supported option is used as the default.

If <varname>fsync</varname> is off then this setting is irrelevant.

This option can only be set at server start or in the

<filename>postgresql.conf</filename> file.