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<Chapter Id="runtime">
<title>Cost-Based Vacuum Delay</title>
<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
+ During the execution of <command>VACUUM</command>
+ and <command>ANALYZE</command> commands,
+ the system maintains an internal counter that keeps track of the
+ estimated 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 process
+ 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 intent of this feature is to allow administrators the reduce
+ The intent of this feature is to allow administrators to 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
+ important that maintenance commands like
<command>VACUUM</command> and <command>ANALYZE</command> finish
- quickly; however, it is usually very important these these
+ quickly; however, it is usually very important that 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>
This feature is disabled by default. To enable it, set the
- <varname>vacuum_cost_naptime</varname> variable to a reasonable
+ <varname>vacuum_cost_naptime</varname> variable to a nonzero
value.
</para>
<term><varname>vacuum_cost_page_hit</varname> (<type>integer</type>)</term>
<listitem>
<para>
- The cost for vacuuming a buffer found in the shared buffer
+ The estimated 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.
<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
+ The estimated 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.
<term><varname>vacuum_cost_page_dirty</varname> (<type>integer</type>)</term>
<listitem>
<para>
- The extra cost added when vacuum modifies a block that was
+ The estimated cost charged 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.
<term><varname>vacuum_cost_limit</varname> (<type>integer</type>)</term>
<listitem>
<para>
- The accumulated cost that will cause the backend to briefly
+ The accumulated cost that will cause the vacuuming process to briefly
nap. The default value is 200.
</para>
</listitem>
<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 length of time, in milliseconds, that the process will nap
+ when the cost limit has been exceeded.
The default value is 0, which disables the cost-based vacuum
- delay feature.
+ delay feature. Positive values enable cost-based vacuuming.
+ Note however that on many systems, the effective resolution
+ of sleep delays is 10 milliseconds; setting
+ <varname>vacuum_cost_naptime</varname> to a value that is
+ not a multiple of 10 may have the same results as setting it
+ to the next higher multiple of 10.
</para>
</listitem>
</varlistentry>
</variablelist>
+
+ <note>
+ <para>
+ There are certain bulk operations that hold critical locks and should
+ therefore complete as quickly as possible. Cost-based vacuum
+ delays do not occur during such operations. Therefore it is
+ possible that the cost accumulates far higher than the specified
+ limit. To avoid uselessly long delays in such cases, the actual
+ 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>
+ </note>
+
</sect3>
</sect2>
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