Add a new WAL summarizer process.

When active, this process writes WAL summary files to
$PGDATA/pg_wal/summaries. Each summary file contains information for a
certain range of LSNs on a certain TLI. For each relation, it stores a
"limit block" which is 0 if a relation is created or destroyed within
a certain range of WAL records, or otherwise the shortest length to
which the relation was truncated during that range of WAL records, or
otherwise InvalidBlockNumber. In addition, it stores a list of blocks
which have been modified during that range of WAL records, but
excluding blocks which were removed by truncation after they were
modified and never subsequently modified again.

In other words, it tells us which blocks need to copied in case of an
incremental backup covering that range of WAL records. But this
doesn't yet add the capability to actually perform an incremental
backup; the next patch will do that.

A new parameter summarize_wal enables or disables this new background
process.  The background process also automatically deletes summary
files that are older than wal_summarize_keep_time, if that parameter
has a non-zero value and the summarizer is configured to run.

Patch by me, with some design help from Dilip Kumar and Andres Freund.
Reviewed by Matthias van de Meent, Dilip Kumar, Jakub Wartak, Peter
Eisentraut, and Álvaro Herrera.

Discussion: http://postgr.es/m/CA+TgmoYOYZfMCyOXFyC-P+-mdrZqm5pP2N7S-r0z3_402h9rsA@mail.gmail.com

1 files changed, 61 insertions, 0 deletions

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 44cada2b403..ee985850275 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -4150,6 +4150,67 @@ restore_command = 'copy "C:\\server\\archivedir\\%f" "%p"'  # Windows
     </variablelist>
    </sect2>
 
+   <sect2 id="runtime-config-wal-summarization">
+    <title>WAL Summarization</title>
+
+ <!--
+    <para>
+     These settings control WAL summarization, a feature which must be
+     enabled in order to perform an
+     <link linkend="backup-incremental-backup">incremental backup</link>.
+    </para>
+ -->
+
+    <variablelist>
+     <varlistentry id="guc-summarize-wal" xreflabel="summarize_wal">
+      <term><varname>summarize_wal</varname> (<type>boolean</type>)
+      <indexterm>
+       <primary><varname>summarize_wal</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+     <listitem>
+      <para>
+       Enables the WAL summarizer process. Note that WAL summarization can
+       be enabled either on a primary or on a standby. WAL summarization
+       cannot be enabled when <varname>wal_level</varname> is set to
+       <literal>minimal</literal>.  This parameter can only be set in the
+       <filename>postgresql.conf</filename> file or on the server command line.
+       The default is <literal>off</literal>.
+      </para>
+      </listitem>
+     </varlistentry>
+
+     <varlistentry id="guc-wal-summary-keep-time" xreflabel="wal_summary_keep_time">
+      <term><varname>wal_summary_keep_time</varname> (<type>boolean</type>)
+      <indexterm>
+       <primary><varname>wal_summary_keep_time</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+     <listitem>
+      <para>
+       Configures the amount of time after which the WAL summarizer
+       automatically removes old WAL summaries. The file timestamp is used to
+       determine which files are old enough to remove. Typically, you should set
+       this comfortably higher than the time that could pass between a backup
+       and a later incremental backup that depends on it.  WAL summaries must
+       be available for the entire range of WAL records between the preceding
+       backup and the new one being taken; if not, the incremental backup will
+       fail. If this parameter is set to zero, WAL summaries will not be
+       automatically deleted, but it is safe to manually remove files that you
+       know will not be required for future incremental backups.
+       This parameter can only be set in the
+       <filename>postgresql.conf</filename> file or on the server command line.
+       The default is 10 days. If <literal>summarize_wal = off</literal>,
+       existing WAL summaries will not be removed regardless of the value of
+       this parameter, because the WAL summarizer will not run.
+      </para>
+      </listitem>
+     </varlistentry>
+
+    </variablelist>
+
+   </sect2>
+
    </sect1>
 
    <sect1 id="runtime-config-replication">