Add documentation chapter about checksums

Data checksums did not have a longer discussion in the docs,
this adds a short section with an overview.

Extracted from the larger patch for on-line enabling of checksums, which
has many more authors and reviewers.

Author: Daniel Gustafsson
Reviewed-By: Magnus Hagander, Michael Banck (and others through the big patch)
Discussion: https://postgr.es/m/5ff49fa4.1c69fb81.658f3.04ac@mx.google.com

doc/src/sgml/amcheck.sgml

@@ -393,7 +393,7 @@ SET client_min_messages = DEBUG1;
  <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>
@@ -497,7 +497,7 @@ SET client_min_messages = DEBUG1;
   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>
 

doc/src/sgml/ref/initdb.sgml

@@ -219,6 +219,7 @@ PostgreSQL documentation
         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>

doc/src/sgml/wal.sgml

@@ -196,7 +196,7 @@
       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 data page checksums.
+      for details about enabling data checksums.
      </para>
     </listitem>
     <listitem>
@@ -230,6 +230,53 @@
   </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>