$PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.237 2004/02/13 12:25:09 wieck Exp $

<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>