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Oracle Tips by Burleson

Disk I/O Tuning

Chapter 3: SCSI Tuning Under AIX

# chdev -l hdisk8 -a q_type=simple -a queue_depth=24

If the disk array is attached via a SCSI-2 Fast/Wide SCSI adapter bus, it may also be necessary to change the outstanding-request limit for that bus.  Let's look at that next.

Changing AIX Disk Adapter Outstanding-Request Limits

The AIX SCSI-2 Fast/Wide Adapter supports two SCSI buses; one for internal devices and one for external devices.  There is a limit on the total number of outstanding requests that can be defined for each bus.  The default value of that limit is 40 for each bus and the maximum is 128.  When an IBM disk array is attached to a SCSI-2 Fast/Wide Adapter bus, the outstanding-request limit for the bus is increased automatically to accommodate the queue depth of the disk array.  However, for a non-IBM disk array, this change must be performed manually.  For example, using chdev to set the outstanding-request limit of adapter scsi2 to 80, you would use:

# chdev -l scsi2 -a num_cmd_elems=80

Note, if you are using the SCSI-2 High Performance Controller, the maximum number of queued requests is 30 and that limit cannot be changed.  For that reason, you should ensure the sum of the queue depths of the devices attached to a SCSI-2 High Performance Controller does not exceed 30.

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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