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Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/sgml/config.sgml | 51 |
1 files changed, 50 insertions, 1 deletions
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml index 0d8cf0a84cb..7d21e31bdb5 100644 --- a/doc/src/sgml/config.sgml +++ b/doc/src/sgml/config.sgml @@ -1,4 +1,4 @@ -<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.204 2009/01/09 10:13:18 mha Exp $ --> +<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.205 2009/01/12 05:10:44 tgl Exp $ --> <chapter Id="runtime-config"> <title>Server Configuration</title> @@ -1203,6 +1203,55 @@ SET ENABLE_SEQSCAN TO OFF; queries. </para> </sect2> + + <sect2 id="runtime-config-resource-async-behavior"> + <title>Asynchronous Behavior</title> + + <variablelist> + <varlistentry id="guc-effective-io-concurrency" xreflabel="effective_io_concurrency"> + <term><varname>effective_io_concurrency</varname> (<type>integer</type>)</term> + <indexterm> + <primary><varname>effective_io_concurrency</> configuration parameter</primary> + </indexterm> + <listitem> + <para> + Sets the number of concurrent disk I/O operations that + <productname>PostgreSQL</> expects can be executed + simultaneously. Raising this value will increase the number of I/O + operations that any individual <productname>PostgreSQL</> session + attempts to initiate in parallel. The allowed range is 1 to 1000, + or zero to disable issuance of asynchronous I/O requests. + </para> + + <para> + A good starting point for this setting is the number of separate + drives comprising a RAID 0 stripe or RAID 1 mirror being used for the + database. (For RAID 5 the parity drive should not be counted.) + However, if the database is often busy with multiple queries issued in + concurrent sessions, lower values may be sufficient to keep the disk + array busy. A value higher than needed to keep the disks busy will + only result in extra CPU overhead. + </para> + + <para> + For more exotic systems, such as memory-based storage or a RAID array + that is limited by bus bandwidth, the correct value might be the + number of I/O paths available. Some experimentation may be needed + to find the best value. + </para> + + <para> + Asynchronous I/O depends on an effective <function>posix_fadvise</> + function, which some operating systems lack. If the function is not + present then setting this parameter to anything but zero will result + in an error. On some operating systems the function is present but + does not actually do anything. On such systems setting a nonzero + value will add CPU overhead without improving performance. + </para> + </listitem> + </varlistentry> + </variablelist> + </sect2> </sect1> <sect1 id="runtime-config-wal"> |