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<Chapter Id="runtime">

</sect3>

<sect3 id="runtime-config-resource-vacuum-cost">

<title>CostBased Vacuum Delay</title>

<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>

During the execution of <command>VACUUM</command>,

<command>VACUUM FULL</command> and <command>ANALYZE</command>,

the system mantains an internal counter that keeps track of the

cost of the various I/O operations that are performed. When the

accumulated cost reaches a limit

(specified by <varname>vacuum_cost_limit</varname>), the backend performing

the operation will sleep for a while (specified by

<varname>vacuum_cost_naptime</varname>). Then it will reset the

counter and continue execution.

</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>

<para>

The intent of this feature is to allow administrators the reduce

the I/O impact of these commands on concurrent database

activity. There are some situations in which it is not very

important that maintainence commands like

<command>VACUUM</command> and <command>ANALYZE</command> finish

quickly; however, it is usually very important these these

commands do not significantly interfere with the ability of the

system to perform other database operations. Cost-based vacuum

delay provides a way for administrators to achieve this.

</para>

<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>

<para>

This feature is disabled by default. To enable it, set the

<varname>vacuum_cost_naptime</varname> variable to a reasonable

value.

</para>

<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>

<variablelist>

<varlistentry>

<term><varname>vacuum_cost_page_hit</varname> (<type>integer</type>)</term>

<listitem>

<para>

The cost for vacuuming a buffer found in the shared buffer

cache. It represents the cost to lock the buffer pool, lookup

the shared hash table and scan the content of the page. The

default value is 1.

</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_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 shared hash table, read the desired block in from

the disk and scan its content. The default value is 10.

</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>

<varlistentry>

<term><varname>vacuum_cost_page_dirty</varname> (<type>integer</type>)</term>

<listitem>

<para>

The extra cost added when vacuum modifies a block that was

previously clean. It represents the extra I/O required to

flush the dirty block out to disk again. The default value is

20.

</para>

</listitem>

</varlistentry>

<varlistentry>

<term><varname>vacuum_cost_limit</varname> (<type>integer</type>)</term>

<listitem>

<para>

The accumulated cost that will cause the backend to briefly

nap. The default value is 200.

</para>

</listitem>

</varlistentry>

<varlistentry>

<term><varname>vacuum_cost_naptime</varname> (<type>integer</type>)</term>

<listitem>

<para>

The length of time in milliseconds that a backend will nap

when the cost limit has been exceeded. There are certain bulk

operations that hold critical locks and should therefore

complete 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>

<para>

The default value is 0, which disables the cost-based vacuum

delay feature.

</para>

</listitem>

</varlistentry>

</variablelist>

</sect3>

</sect2>