<para>

<filename>amcheck</filename> can be effective at detecting various types of

failure modes that <link

linkend="app-initdb-data-checksums"><application>data page

linkend="app-initdb-data-checksums"><application>data

checksums</application></link> will fail to catch. These include:

<itemizedlist>

Structural corruption can happen due to faulty storage hardware, or

relation files being overwritten or modified by unrelated software.

This kind of corruption can also be detected with

<link linkend="app-initdb-data-checksums"><application>data page

<link linkend="checksums"><application>data page

checksums</application></link>.

</para>

failures will be reported in the

<link linkend="monitoring-pg-stat-database-view">

<structname>pg_stat_database</structname></link> view.

See <xref linkend="checksums" /> for details.

</para>

</listitem>

</varlistentry>

Data pages are not currently checksummed by default, though full page images

recorded in WAL records will be protected; see <link

linkend="app-initdb-data-checksums"><application>initdb</application></link>

for details about enabling datapagechecksums.

for details about enabling data checksums.

</para>

</listitem>

<listitem>

</para>

</sect1>

<sect1 id="checksums">

<title>Data Checksums</title>

<indexterm>

<primary>checksums</primary>

</indexterm>

<para>

Data pages are not checksum protected by default, but this can optionally be

enabled for a cluster. When enabled, each data page will be assigned a

checksum that is updated when the page is written and verified every time

the page is read. Only data pages are protected by checksums, internal data

structures and temporary files are not.

</para>

<para>

Checksums are normally enabled when the cluster is initialized using <link

linkend="app-initdb-data-checksums"><application>initdb</application></link>.

They can also be enabled or disabled at a later time as an offline

operation. Data checksums are enabled or disabled at the full cluster

level, and cannot be specified individually for databases or tables.

</para>

<para>

The current state of checksums in the cluster can be verified by viewing the

value of the read-only configuration variable <xref

linkend="guc-data-checksums" /> by issuing the command <command>SHOW

data_checksums</command>.

</para>

<para>

When attempting to recover from corrupt data it may be necessary to bypass

the checksum protection in order to recover data. To do this, temporarily

set the configuration parameter <xref linkend="guc-ignore-checksum-failure" />.

</para>

<sect2 id="checksums-offline-enable-disable">

<title>Off-line Enabling of Checksums</title>

<para>

The <link linkend="app-pgchecksums"><application>pg_checksums</application></link>

application can be used to enable or disable data checksums, as well as

verify checksums, on an offline cluster.

</para>

</sect2>

</sect1>

<sect1 id="wal-intro">

<title>Write-Ahead Logging (<acronym>WAL</acronym>)</title>