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Oracle Tips by Burleson

Disk I/O Tuning

Chapter 3: HP-UX SCSI Tuning

This value should only be raised in SCSI devices that have enough memory to support higher queue depth than the default set by HP.  Such devices may offer better performance if the queue depth is set to a higher value.

Remember,  the queue depth applies to all the SCSI devices that support tag queuing.  Setting the queue depth to a value larger than the disk can handle will result in I/Os being held off once a QUEUE FULL condition exists on the disk.  A mechanism exists that will lower the queue depth of the device in case of QUEUE FULL condition avoiding infinite QUEUE FULL conditions on that device similar to the process that lowers it to 1 on SUN Solaris.  Nevertheless, this mechanism will periodically try higher queue depths and QUEUE FULL conditions will arise.

This parameter should be lowered when the connected SCSI devices support smaller queue depth or for load balancing.  However, devices that support higher queue depth may not deliver optimal performance when a lower queue depth value is set.

The other tunable parameter in HP-UX for SCSI is scsi_maxphys, which sets the maximum allowed length of an IO on all SCSI devices.  scsi_maxphys has a minimum and maximum setting of 1048576 (one megabyte) except on V-class systems, where it has the range of values: 16777215 and 33554432.

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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