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Add a paragraph explaining what restartpoints are. Mention that 

wal_keep_segments does not take effect during recovery.

Fujii Masao
This commit is contained in:
Heikki Linnakangas 2010-07-16 11:20:23 +00:00
parent 7590ddb3eb
commit 8f9c461175
2 changed files with 21 additions and 3 deletions
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5
doc/src/sgml/config.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.294 2010/07/08 10:20:13 mha Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.295 2010/07/16 11:20:23 heikki Exp $ -->
<chapter Id="runtime-config">
<title>Server Configuration</title>
@ -1926,7 +1926,8 @@ SET ENABLE_SEQSCAN TO OFF;
doesn't keep any extra segments for standby purposes, and the number
of old WAL segments available to standby servers is a function of
the location of the previous checkpoint and status of WAL
archiving. This parameter can only be set in the
archiving. This parameter has no effect on restartpoints.
This parameter can only be set in the
<filename>postgresql.conf</> file or on the server command line.
</para>
</listitem>

19
doc/src/sgml/wal.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/wal.sgml,v 1.68 2010/07/08 16:44:12 momjian Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/wal.sgml,v 1.69 2010/07/16 11:20:23 heikki Exp $ -->
<chapter id="wal">
<title>Reliability and the Write-Ahead Log</title>
@ -449,6 +449,7 @@
<para>
There will always be at least one WAL segment file, and will normally
not be more than (2 + <varname>checkpoint_completion_target</varname>) * <varname>checkpoint_segments</varname> + 1
or <varname>checkpoint_segments</> + <xref linkend="guc-wal-keep-segments"> + 1
files. Each segment file is normally 16 MB (though this size can be
altered when building the server). You can use this to estimate space
requirements for <acronym>WAL</acronym>.
@ -460,6 +461,22 @@
of recycled until the system gets back under this limit.
</para>
<para>
In archive recovery or standby mode, the server periodically performs
<firstterm>restartpoints</><indexterm><primary>restartpoint</></>
which are similar to checkpoints in normal operation: the server forces
all its state to disk, updates the <filename>pg_control</> file to
indicate that the already-processed WAL data need not be scanned again,
and then recycles any old log segment files in <filename>pg_xlog</>
directory. A restartpoint is triggered if at least one checkpoint record
has been replayed and <varname>checkpoint_timeout</> seconds have passed
since last restartpoint. In standby mode, a restartpoint is also triggered
if <varname>checkoint_segments</> log segments have been replayed since
last restartpoint and at least one checkpoint record has been replayed.
Restartpoints can't be performed more frequently than checkpoints in the
master because restartpoints can only be performed at checkpoint records.
</para>
<para>
There are two commonly used internal <acronym>WAL</acronym> functions:
<function>LogInsert</function> and <function>LogFlush</function>.