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<chapter Id="runtime-config">

<title>Server Configuration</title>

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>

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<chapter id="wal">

<title>Reliability and the Write-Ahead Log</title>

<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>.

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>.