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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

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Burleson Oracle Consulting offers Senior Oracle consultants which are available for all areas of Oracle support.  Common Oracle consulting support activities include short-term Oracle tuning, Oracle database troubleshooting, Oracle9i and Oracle10g migration, Oracle9iAS App Server consulting, Oracle design reviews and Oracle requirements evaluation support. Oracle support and Oracle consulting services are priced by the hour, so you only pay for what you need. These one-time Oracle consulting services commonly include:

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