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Significant improvements to the documentation for the new cost-based
vacuum delay feature, including updating the docs for Tom's recent improvements. There is still more work to be done here: for example, adding some more information on the practical use of cost-based vacuum delay to the "maintenance" section would probably be a good idea.
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Neil Conway
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<!--
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$PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.239 2004/02/17 05:45:17 neilc Exp $
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$PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.240 2004/02/17 06:28:05 neilc Exp $
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-->
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<Chapter Id="runtime">
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@ -992,83 +992,107 @@ SET ENABLE_SEQSCAN TO OFF;
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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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<title>Cost-Based Vacuum Delay</title>
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<para>
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During the execution of <command>VACUUM</command>,
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<command>VACUUM FULL</command> and <command>ANALYZE</command>,
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the system mantains an internal counter that keeps track of the
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cost of the various I/O operations that are performed. When the
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accumulated cost reaches a limit
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(specified by <varname>vacuum_cost_limit</varname>), the backend performing
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the operation will sleep for a while (specified by
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<varname>vacuum_cost_naptime</varname>). Then it will reset the
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counter and continue execution.
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</para>
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<para>
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The intent of this feature is to allow administrators the reduce
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the I/O impact of these commands on concurrent database
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activity. There are some situations in which it is not very
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important that maintainence commands like
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<command>VACUUM</command> and <command>ANALYZE</command> finish
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quickly; however, it is usually very important these these
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commands do not significantly interfere with the ability of the
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system to perform other database operations. Cost-based vacuum
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delay provides a way for administrators to achieve this.
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</para>
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<para>
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This feature is disabled by default. To enable it, set the
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<varname>vacuum_cost_naptime</varname> variable to a reasonable
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value.
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</para>
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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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<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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The cost for vacuuming a buffer found in the shared buffer
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cache. It represents the cost to lock the buffer pool, lookup
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the shared hash table and scan the content of the page. The
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default value is 1.
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</para>
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</listitem>
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</varlistentry>
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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
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disk. This represents the effort to lock the buffer pool,
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lookup the shared hash table, read the desired block in from
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the disk and scan its content. The default value is 10.
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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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The extra cost added when vacuum modifies a block that was
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previously clean. It represents the extra I/O required to
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flush the dirty block out to disk again. The default value is
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20.
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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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The accumulated cost that will cause the backend to briefly
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nap. The default value is 200.
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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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<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 length of time in milliseconds that a backend will nap
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when the cost limit has been exceeded. There are certain bulk
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operations that hold critical locks and should therefore
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complete as quickly as possible. Because of that it is
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possible that the cost actually accumulates far higher than
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this limit. To compensate for this, the final naptime is
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calculated as <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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<para>
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The default value is 0, which disables the cost-based vacuum
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delay feature.
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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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