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Minor editorializing on cost-based vacuum description.

This commit is contained in:
Tom Lane 2004-02-17 07:36:47 +00:00
parent ee33fe889e
commit d46b1f904e
1 changed files with 39 additions and 30 deletions
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69
doc/src/sgml/runtime.sgml
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@ -1,5 +1,5 @@
<!--
$PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.240 2004/02/17 06:28:05 neilc Exp $
$PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.241 2004/02/17 07:36:47 tgl Exp $
-->
<Chapter Id="runtime">
@ -995,24 +995,24 @@ SET ENABLE_SEQSCAN TO OFF;
<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.
@ -1020,7 +1020,7 @@ SET ENABLE_SEQSCAN TO OFF;
<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>
@ -1029,7 +1029,7 @@ SET ENABLE_SEQSCAN TO OFF;
<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.
@ -1041,7 +1041,7 @@ SET ENABLE_SEQSCAN TO OFF;
<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.
@ -1053,7 +1053,7 @@ SET ENABLE_SEQSCAN TO OFF;
<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.
@ -1065,7 +1065,7 @@ SET ENABLE_SEQSCAN TO OFF;
<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>
@ -1075,25 +1075,34 @@ SET ENABLE_SEQSCAN TO OFF;
<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>