</itemizedlist>

<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. 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 default is the first method in the above list that is supported

by the platform. The default is not necessarily ideal; it might be

necessary to change this setting or other aspects of your system

configuration in order to create a crash-safe configuration or

achieve optimal performance.

These aspects are discussed in <xref linkend="wal-reliability">.

The utility <filename>src/tools/fsync</> in the PostgreSQL source tree

can do performance testing of various fsync methods.

This parameter can only be set in the <filename>postgresql.conf</>

<para>

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.

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.

<acronym>WAL</acronym> updates out to disk.

All the options should be the same in terms of reliability, 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.

However, it's quite platform-specific which one will be the fastest;

you can test option speeds using the utility <filename>src/tools/fsync</>

in the PostgreSQL source tree.

Note that this parameter is irrelevant if <varname>fsync</varname>

has been turned off.

</para>