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Add a note that pg_start_backup will take awhile because of new

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distributed checkpoint behavior.  Explain how to work around this
by issuing a manual CHECKPOINT command.  Per discussion with Heikki.
This commit is contained in:
Tom Lane
2007-06-29 15:46:21 +00:00
parent 6faf795662
commit 4303c0fdbf
1 changed files with 17 additions and 1 deletions
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18
doc/src/sgml/backup.sgml
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@@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/backup.sgml,v 2.97 2007/02/01 00:28:16 momjian Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/backup.sgml,v 2.98 2007/06/29 15:46:21 tgl Exp $ -->
<chapter id="backup">
<title>Backup and Restore</title>
@@ -674,6 +674,22 @@ SELECT pg_start_backup('label');
issue this command. You can ignore the result returned by the function;
but if it reports an error, deal with that before proceeding.
</para>
<para>
<function>pg_start_backup</> can take a long time to finish.
This is because it performs a checkpoint, and the I/O
required for a checkpoint will be spread out over a significant
period of time, by default half your inter-checkpoint interval
(see the configuration parameter
<xref linkend="guc-checkpoint-completion-target">). Usually
this is what you want because it minimizes the impact on query
processing. If you just want to start the backup as soon as
possible, execute a <command>CHECKPOINT</> command
(which performs a checkpoint as quickly as possible) and then
immediately execute <function>pg_start_backup</>. Then there
will be very little for <function>pg_start_backup</>'s checkpoint
to do, and it won't take long.
</para>
</listitem>
<listitem>
<para>