From 8f9c46117569131e63f36dbd5412f68d9ab167b2 Mon Sep 17 00:00:00 2001
From: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date: Fri, 16 Jul 2010 11:20:23 +0000
Subject: [PATCH] Add a paragraph explaining what restartpoints are. Mention
that wal_keep_segments does not take effect during recovery.
Fujii Masao
---
doc/src/sgml/config.sgml | 5 +++--
doc/src/sgml/wal.sgml | 19 ++++++++++++++++++-
2 files changed, 21 insertions(+), 3 deletions(-)
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 2fbe6a8c24b..9890f9f6f4e 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -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>
diff --git a/doc/src/sgml/wal.sgml b/doc/src/sgml/wal.sgml
index 0c83032968a..cf580e9f45e 100644
--- a/doc/src/sgml/wal.sgml
+++ b/doc/src/sgml/wal.sgml
@@ -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>.
--
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