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Oracle Tips by Burleson |
Disk I/O Tuning
Chapter 3:
HP-UX SCSI Tuning
Queue
depth is synonymous to the tagged command queuing in other systems.
When tagged command queuing is supported by a target, it allows the
target to accept multiple SCSI commands for execution. By accepting
multiple commands, intelligent on-disk controllers can optimize how
the commands are carried out by positioning reads and writes in the
order to minimize disk head movement between operations. Some
targets can allow up to 256 commands to be stored from different
initiators. Once the target command queue is full, the target
terminates any additional I/O and returns a QUEUE FULL status to the
initiator. Some older targets may support less than 256 commands to
be queued, and this is the reason the setting defaults to 8. Why
the value is set so low is not explained.
If the system has a
combination of devices that support small and larger queue depths,
then a queue depth can be set to a value, which would work for most
devices. For specific devices, the system administrator can change
the queue depth on a per device basis using
SIO_SET_LUN_LIMIT in
ioctl().
See the HP-UX manual entries on scsictl(1M) for more on how
to use
ioctl().
The above text is
an excerpt from:
Oracle Disk I/O Tuning
Disk IO Performance & Optimization for Oracle
Databases
ISBN
0-9745993-4-4
by Mike Ault
Oracle Consulting
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