|The PostgreSQL 9.0 Reference Manual - Volume 3 - Server Administration Guide
by The PostgreSQL Global Development Group
Paperback (6"x9"), 274 pages
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4.4.5 Asynchronous Behavior
Sets the number of concurrent disk I/O operations that
PostgreSQL expects can be executed
simultaneously. Raising this value will increase the number of I/O
operations that any individual PostgreSQL session
attempts to initiate in parallel. The allowed range is 1 to 1000,
or zero to disable issuance of asynchronous I/O requests.
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.
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.
Asynchronous I/O depends on an effective
posix_fadvisefunction, 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 (e.g., Solaris), the function is present but does not actually do anything.
|ISBN 9781906966072||The PostgreSQL 9.0 Reference Manual - Volume 3 - Server Administration Guide||See the print edition|