Add archive_mode='always' option.

In 'always' mode, the standby independently archives all files it receives
from the primary.

Original patch by Fujii Masao, docs and review by me.

2 files changed, 50 insertions, 2 deletions

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 0d8624af718..5549b7dce62 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -2521,7 +2521,7 @@ include_dir 'conf.d'
 
     <variablelist>
      <varlistentry id="guc-archive-mode" xreflabel="archive_mode">
-      <term><varname>archive_mode</varname> (<type>boolean</type>)
+      <term><varname>archive_mode</varname> (<type>enum</type>)
       <indexterm>
        <primary><varname>archive_mode</> configuration parameter</primary>
       </indexterm>
@@ -2530,7 +2530,16 @@ include_dir 'conf.d'
        <para>
         When <varname>archive_mode</> is enabled, completed WAL segments
         are sent to archive storage by setting
-        <xref linkend="guc-archive-command">.
+        <xref linkend="guc-archive-command">. In addition to <literal>off</>,
+        to disable, there are two modes: <literal>on</>, and
+        <literal>always</>. During normal operation, there is no
+        difference between the two modes, but when set to <literal>always</>
+        the WAL archiver is enabled also during archive recovery or standby
+        mode. In <literal>always</> mode, all files restored from the archive
+        or streamed with streaming replication will be archived (again). See
+        <xref linkend="continuous-archiving-in-standby"> for details.
+       </para>  
+       <para>
         <varname>archive_mode</> and <varname>archive_command</> are
         separate variables so that <varname>archive_command</> can be
         changed without leaving archiving mode.
diff --git a/doc/src/sgml/high-availability.sgml b/doc/src/sgml/high-availability.sgml
index a17f55545c9..46518262cd6 100644
--- a/doc/src/sgml/high-availability.sgml
+++ b/doc/src/sgml/high-availability.sgml
@@ -1220,6 +1220,45 @@ primary_slot_name = 'node_a_slot'
 
    </sect3>
   </sect2>
+
+  <sect2 id="continuous-archiving-in-standby">
+   <title>Continuous archiving in standby</title>
+
+   <indexterm>
+     <primary>continuous archiving</primary>
+     <secondary>in standby</secondary>
+   </indexterm>
+
+   <para>
+     When continuous WAL archiving is used in a standby, there are two
+     different scenarios: the WAL archive can be shared between the primary
+     and the standby, or the standby can have its own WAL archive. When
+     the standby has its own WAL archive, set <varname>archive_mode</varname>
+     to <literal>always</literal>, and the standby will call the archive
+     command for every WAL segment it receives, whether it's by restoring
+     from the archive or by streaming replication. The shared archive can
+     be handled similarly, but the archive_command must test if the file
+     being archived exists already, and if the existing file has identical
+     contents. This requires more care in the archive_command, as it must
+     be careful to not overwrite an existing file with different contents,
+     but return success if the exactly same file is archived twice. And
+     all that must be done free of race conditions, if two servers attempt
+     to archive the same file at the same time.
+   </para>
+
+   </para>
+     If <varname>archive_mode</varname> is set to <literal>on</>, the
+     archiver is not enabled during recovery or standby mode. If the standby
+     server is promoted, it will start archiving after the promotion, but
+     will not archive any WAL it did not generate itself. To get a complete
+     series of WAL files in the archive, you must ensure that all WAL is
+     archived, before it reaches the standby. This is inherently true with
+     file-based log shipping, as the standby can only restore files that
+     are found in the archive, but not if streaming replication is enabled.
+     When a server is not in recovery mode, there is no difference between
+     <literal>on</literal> and <literal>always</literal> modes.
+   </para>
+  </sect2>
   </sect1>
 
   <sect1 id="warm-standby-failover">