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Oracle Tips by Burleson

Disk I/O Tuning

Chapter 3: SCSI Tuning Under AIX

fast-pathing to specific points in the SMIT interface from the command line.  The fast path is smitty chgdsk to get the disk interface section of SMIT.  As an alternative, you can also use the chdev command line command to change these parameters.

Let's look at an example using chdev, if your system contained a non-IBM SCSI disk drive hdisk7, the following command enables queuing for that device and sets its queue depth to 3:

# chdev -l hdisk7 -a q_type=simple -a queue_depth=3

That's fine for a single disk, but what about when you are dealing with an entire non-IBM disk array?  Let's look at that next.

Setting SCSI Parameters for a Non-IBM Disk Array

Any disk array appears to AIX as a single, rather large, disk drive.  A non-IBM disk array, like a non-IBM disk drive, will be of class disk, subclass scsi, type osdisk (which is short for "Other SCSI Disk Drive").  However, we know a disk array actually contains a number of physical disk drives.  Each physical disk drive can handle multiple requests, therefore the queue depth for the disk array device has to be set to a value high enough to allow efficient use of all of the physical devices.  For example, if hdisk8 were an eight-disk non-IBM disk array, where each disk supports a queued depth of 3, an appropriate change using chdev would be:

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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