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Documentation for vacuum_cost config options.

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Jan Wieck 2004-02-13 12:25:09 +00:00
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doc/src/sgml/runtime.sgml
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<!--
$PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.236 2004/02/03 17:34:02 tgl Exp $
$PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.237 2004/02/13 12:25:09 wieck Exp $
-->
<Chapter Id="runtime">
@ -990,6 +990,87 @@ SET ENABLE_SEQSCAN TO OFF;
</variablelist>
</sect3>
<sect3 id="runtime-config-resource-vacuum-cost">
<title>Cost Based Vacuum Delay</title>
<variablelist>
<varlistentry>
<term><varname>vacuum_cost_page_hit</varname> (<type>integer</type>)</term>
<listitem>
<para>
During a default <command>VACUUM</command> (not
<command>FULL</command>) the system maintains and internal counter
accumulating the cost of various operations performed. When the
accumulated cost reaches a limit, the backend performing the
<command>VACUUM</command> will sleep for a while, reset the
accumulator and continue. The intention is to lower the IO impact
of <command>VACUUM</command>.
</para>
<para>
The variable <varname>vacuum_cost_page_hit</varname> is the cost
for vacuuming a buffer found inside the shared buffer cache.
It represents the cost to lock the buffer pool, lookup the
shared hash table and to actually scan the block content.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>vacuum_cost_page_miss</varname> (<type>integer</type>)</term>
<listitem>
<para>
The cost for vacuuming a buffer that has to be read from disk.
This represents the effort to lock the buffer pool, lookup the
cache directory, reading the block from disk and scanning the
content.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>vacuum_cost_page_dirty</varname> (<type>integer</type>)</term>
<listitem>
<para>
This extra cost is added when vacuum modifies a block that was
clean before. It represents the extra IO required to flush the
dirty block out to disk again.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>vacuum_cost_limit</varname> (<type>integer</type>)</term>
<listitem>
<para>
This is the cost limit that must be reached or exceeded before
the <command>VACUUM</command> will nap.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>vacuum_cost_naptime</varname> (<type>integer</type>)</term>
<listitem>
<para>
The time im milliseconds the <command>VACUUM</command> will
nap when the cost limit has been reached or exceeded.
There are certain bulk operations that hold critical
locks and should therefore perform
as quickly as possible. Because of that it is possible that the
cost actually accumulates far higher than this limit. To compensate
for this, the final naptime is calculated as
<varname>vacuum_cost_naptime</varname> *
<varname>accumulated_balance</varname> /
<varname>vacuum_cost_limit</varname> with a maximum of
<varname>vacuum_cost_naptime</varname> * 4.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect3>
</sect2>
<sect2 id="runtime-config-wal">