</term>

<listitem>

<para>

Sets the number of concurrentdisk I/O operations that

Sets the number of concurrentstorage I/O operations that

<productname>PostgreSQL</productname> expects can be executed

simultaneously. Raising this value will increase the number of I/O

operations that any individual <productname>PostgreSQL</productname> session

attempts to initiate in parallel. The allowed range is 1 to 1000,

or zero to disable issuance of asynchronous I/O requests. Currently,

this setting only affects bitmap heap scans.

operations that any individual <productname>PostgreSQL</productname>

session attempts to initiate in parallel. The allowed range is

<literal>1</literal> to <literal>1000</literal>, or

<literal>0</literal> to disable issuance of asynchronous I/O requests.

The default is <literal>16</literal> on supported systems, otherwise

<literal>0</literal>.

</para>

<para>

For magnetic drives, 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.

SSDs and other memory-based storage can often process many

concurrent requests, so the best value might be in the hundreds.

Higher values will have the most impact on higher latency storage

where queries otherwise experience noticeable I/O stalls and on

devices with high IOPs. Unnecessarily high values may increase I/O

latency for all queries on the system

</para>

<para>

Asynchronous I/O requires that the operating system supports issuing

read-ahead advice. If there is no operating system support then

setting this parameter to anything but zero will result in an error.

On systems without prefetch advice support, attempting to configure

any value other than <literal>0</literal> will error out.

</para>

<para>

The default is 1 on supported systems, otherwise 0. This value can

be overridden for tables in a particular tablespace by setting the

tablespace parameter of the same name (see

<xref linkend="sql-altertablespace"/>).

This value can be overridden for tables in a particular tablespace by

setting the tablespace parameter of the same name (see <xref

linkend="sql-altertablespace"/>).

</para>

</listitem>

</varlistentry>

# - I/O -

#backend_flush_after = 0# measured in pages, 0 disables

#effective_io_concurrency =1# 1-1000; 0 disables prefetching

#effective_io_concurrency =16# 1-1000; 0 disables prefetching

#maintenance_io_concurrency = 10# 1-1000; 0 disables prefetching

#io_combine_limit = 128kB# usually 1-32 blocks (depends on OS)

#defineDEFAULT_MAINTENANCE_IO_CONCURRENCY 10

#defineDEFAULT_EFFECTIVE_IO_CONCURRENCY 0