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Auto-tune effective_cache size to be 4x shared buffers

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This commit is contained in:
Bruce Momjian
2013-10-08 12:12:24 -04:00
parent d29a031926
commit ee1e5662d8
8 changed files with 75 additions and 11 deletions
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22
doc/src/sgml/config.sgml
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@ -2758,7 +2758,7 @@ include 'filename'
<para>
Random access to mechanical disk storage is normally much more expensive
than four-times sequential access. However, a lower default is used
than four times sequential access. However, a lower default is used
(4.0) because the majority of random accesses to disk, such as indexed
reads, are assumed to be in cache. The default value can be thought of
as modeling random access as 40 times slower than sequential, while
@ -2841,9 +2841,17 @@ include 'filename'
<listitem>
<para>
Sets the planner's assumption about the effective size of the
disk cache that is available to a single query. This is
factored into estimates of the cost of using an index; a
higher value makes it more likely index scans will be used, a
disk cache that is available to a single query. The default
setting of -1 selects a size equal to four times the size of <xref
linkend="guc-shared-buffers">, but not less than the size of one
shared buffer page, typically <literal>8kB</literal>. This value
can be set manually if the automatic choice is too large or too
small.
</para>
<para>
This value is factored into estimates of the cost of using an index;
a higher value makes it more likely index scans will be used, a
lower value makes it more likely sequential scans will be
used. When setting this parameter you should consider both
<productname>PostgreSQL</productname>'s shared buffers and the
@ -2855,8 +2863,10 @@ include 'filename'
memory allocated by <productname>PostgreSQL</productname>, nor
does it reserve kernel disk cache; it is used only for estimation
purposes. The system also does not assume data remains in
the disk cache between queries. The default is 128 megabytes
(<literal>128MB</>).
the disk cache between queries. The auto-tuning
selected by the default setting of -1 should give reasonable
results if this database cluster is can utilize most of the memory
on this server.
</para>
</listitem>
</varlistentry>