<!-- $PostgreSQL: pgsql/doc/src/sgml/high-availability.sgml,v 1.80 2010/08/24 15:22:12 momjian Exp $ -->

<sect1 id="backup-incremental-updated">

<title>Incrementally Updated Backups</title>

<indexterm zone="high-availability">

<primary>incrementally updated backups</primary>

</indexterm>

<indexterm zone="high-availability">

<primary>change accumulation</primary>

</indexterm>

<para>

In a standby configuration, it is possible to offload the expense of

taking periodic base backups from the primary server; instead base backups

can be made by backing

up a standby server's files. This concept is generally known as

incrementally updated backups, log change accumulation, or more simply,

change accumulation.

</para>

<para>

If we take a file system backup of the standby server's data

directory while it is processing

logs shipped from the primary, we will be able to reload that backup and

restart the standby's recovery process from the last restart point.

We no longer need to keep WAL files from before the standby's restart point.

If recovery is needed, it will be faster to recover from the incrementally

updated backup than from the original base backup.

</para>

<para>

The procedure for taking a file system backup of the standby server's

data directory while it's processing logs shipped from the primary is:

<orderedlist>

<listitem>

<para>

Perform the backup, without using <function>pg_start_backup</> and

<function>pg_stop_backup</>. Note that the <filename>pg_control</>

file must be backed up <emphasis>first</>, as in:

<programlisting>

cp /var/lib/pgsql/data/global/pg_control /tmp

cp -r /var/lib/pgsql/data /path/to/backup

mv /tmp/pg_control /path/to/backup/data/global

</programlisting>

<filename>pg_control</> contains the location where WAL replay will

begin after restoring from the backup; backing it up first ensures

that it points to the last restartpoint when the backup started, not

some later restartpoint that happened while files were copied to the

backup.

</para>

</listitem>

<listitem>

<para>

Make note of the backup ending WAL location by calling the <function>

pg_last_xlog_replay_location</> function at the end of the backup,

and keep it with the backup.

<programlisting>

psql -c "select pg_last_xlog_replay_location();" > /path/to/backup/end_location

</programlisting>

When recovering from the incrementally updated backup, the server

can begin accepting connections and complete the recovery successfully

before the database has become consistent. To avoid that, you must

ensure the database is consistent before users try to connect to the

server and when the recovery ends. You can do that by comparing the

progress of the recovery with the stored backup ending WAL location:

the server is not consistent until recovery has reached the backup end

location. The progress of the recovery can also be observed with the

<function>pg_last_xlog_replay_location</> function, though that requires

connecting to the server while it might not be consistent yet, so

care should be taken with that method.

</para>

<para>

</para>

</listitem>

</orderedlist>

</para>

<para>

Since the standby server is not <quote>live</>, it is not possible to

use <function>pg_start_backup()</> and <function>pg_stop_backup()</>

to manage the backup process; it will be up to you to determine how

far back you need to keep WAL segment files to have a recoverable

backup. That is determined by the last restartpoint when the backup

was taken, any WAL older than that can be deleted from the archive

once the backup is complete. You can determine the last restartpoint

by running <application>pg_controldata</> on the standby server before

taking the backup, or by using the <varname>log_checkpoints</> option

to print values to the standby's server log.

</para>

</sect1>