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Commitee7769b

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Update docs as to when WAL logging can be skipped.
In 8.4 and prior, WAL-logging could potentially be skipped wheneverarchive_mode=off. With streaming replication, this is now true onlyif max_wal_senders=0.Fujii Masao, with light copyediting by me
1 parentcfac702 commitee7769b

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‎doc/src/sgml/backup.sgml

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<!-- $PostgreSQL: pgsql/doc/src/sgml/backup.sgml,v 2.151 2010/04/12 19:08:28 momjian Exp $ -->
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<!-- $PostgreSQL: pgsql/doc/src/sgml/backup.sgml,v 2.152 2010/04/20 00:26:06 rhaas Exp $ -->
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<chapter id="backup">
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<title>Backup and Restore</title>
@@ -689,13 +689,14 @@ archive_command = 'test ! -f /mnt/server/archivedir/%f &amp;&amp; cp %p /mnt/ser
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</para>
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<para>
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When <varname>archive_mode</> is <literal>off</> some SQL commands
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When <varname>archive_mode</> is <literal>off</> and <xref
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linkend="guc-max-wal-senders"> is zero some SQL commands
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are optimized to avoid WAL logging, as described in <xref
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linkend="populate-pitr">. If archivingwere turned on during execution
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of one of these statements, WAL would not contain enough information
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for archive recovery. (Crash recovery is unaffected.) For
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this reason,<varname>archive_mode</>can only be changed at server
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start. However, <varname>archive_command</> can be changed with a
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linkend="populate-pitr">.If archivingor streaming replication were
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turned on during executionof one of these statements, WAL would not
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contain enough informationfor archive recovery. (Crash recovery is
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unaffected.) Forthis reason,these parameterscan only be changed at
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serverstart. However, <varname>archive_command</> can be changed with a
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configuration file reload. If you wish to temporarily stop archiving,
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one way to do it is to set <varname>archive_command</> to the empty
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string (<literal>''</>).

‎doc/src/sgml/perform.sgml

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<!-- $PostgreSQL: pgsql/doc/src/sgml/perform.sgml,v 1.75 2010/04/03 07:22:55 petere Exp $ -->
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<!-- $PostgreSQL: pgsql/doc/src/sgml/perform.sgml,v 1.76 2010/04/20 00:26:06 rhaas Exp $ -->
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<chapter id="performance-tips">
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<title>Performance Tips</title>
@@ -910,24 +910,28 @@ SELECT * FROM x, y, a, b, c WHERE something AND somethingelse;
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</sect2>
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<sect2 id="populate-pitr">
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<title>Turn off <varname>archive_mode</varname></title>
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<title>Turn off <varname>archive_mode</varname> and streaming replication</title>
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<para>
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When loading large amounts of data into an installation that uses
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WAL archiving, you might want to disable archiving (turn off the
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<xref linkend="guc-archive-mode"> configuration variable)
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WAL archiving or streaming replication, you might want to disable
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archiving (turn off the <xref linkend="guc-archive-mode">
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configuration variable) and replication (zero the
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<xref linkend="guc-max-wal-senders"> configuration variable)
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while loading. It might be
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faster to take a new base backup after the load has completed
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than to process a large amount of incremental WAL data.
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But note thatturning <varname>archive_mode</varname> on or off
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requiresa server restart.
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But note thatchanging either of these variables requires
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a server restart.
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</para>
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<para>
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Aside from avoiding the time for the archiver to process the WAL data,
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Aside from avoiding the time for the archiver or WAL sender to
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process the WAL data,
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doing this will actually make certain commands faster, because they
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are designed not to write WAL at all if <varname>archive_mode</varname>
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is off. (They can guarantee crash safety more cheaply by doing an
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is off and <varname>max_wal_senders</varname> is zero. (They can
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guarantee crash safety more cheaply by doing an
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<function>fsync</> at the end than by writing WAL.)
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This applies to the following commands:
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<itemizedlist>

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