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Commitf884090

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Jan WieckJan Wieck
Jan Wieck
authored and
Jan Wieck
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Documentation for vacuum_cost config options.
Jan
1 parent8787bc8 commitf884090

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‎doc/src/sgml/runtime.sgml

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<!--
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$PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.236 2004/02/03 17:34:02 tgl Exp $
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$PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.237 2004/02/13 12:25:09 wieck Exp $
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-->
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<Chapter Id="runtime">
@@ -990,6 +990,87 @@ SET ENABLE_SEQSCAN TO OFF;
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</variablelist>
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</sect3>
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<sect3 id="runtime-config-resource-vacuum-cost">
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<title>Cost Based Vacuum Delay</title>
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<variablelist>
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<varlistentry>
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<term><varname>vacuum_cost_page_hit</varname> (<type>integer</type>)</term>
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<listitem>
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<para>
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During a default <command>VACUUM</command> (not
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<command>FULL</command>) the system maintains and internal counter
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accumulating the cost of various operations performed. When the
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accumulated cost reaches a limit, the backend performing the
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<command>VACUUM</command> will sleep for a while, reset the
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accumulator and continue. The intention is to lower the IO impact
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of <command>VACUUM</command>.
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</para>
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<para>
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The variable <varname>vacuum_cost_page_hit</varname> is the cost
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for vacuuming a buffer found inside the shared buffer cache.
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It represents the cost to lock the buffer pool, lookup the
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shared hash table and to actually scan the block content.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>vacuum_cost_page_miss</varname> (<type>integer</type>)</term>
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<listitem>
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<para>
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The cost for vacuuming a buffer that has to be read from disk.
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This represents the effort to lock the buffer pool, lookup the
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cache directory, reading the block from disk and scanning the
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content.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>vacuum_cost_page_dirty</varname> (<type>integer</type>)</term>
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<listitem>
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<para>
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This extra cost is added when vacuum modifies a block that was
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clean before. It represents the extra IO required to flush the
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dirty block out to disk again.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>vacuum_cost_limit</varname> (<type>integer</type>)</term>
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<listitem>
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<para>
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This is the cost limit that must be reached or exceeded before
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the <command>VACUUM</command> will nap.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>vacuum_cost_naptime</varname> (<type>integer</type>)</term>
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<listitem>
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<para>
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The time im milliseconds the <command>VACUUM</command> will
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nap when the cost limit has been reached or exceeded.
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There are certain bulk operations that hold critical
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locks and should therefore perform
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as quickly as possible. Because of that it is possible that the
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cost actually accumulates far higher than this limit. To compensate
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for this, the final naptime is calculated as
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<varname>vacuum_cost_naptime</varname> *
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<varname>accumulated_balance</varname> /
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<varname>vacuum_cost_limit</varname> with a maximum of
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<varname>vacuum_cost_naptime</varname> * 4.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</sect3>
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</sect2>
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<sect2 id="runtime-config-wal">

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