Non-IBM disk drive

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For IBM disk drives, the default number of requests that can be outstanding at any given time is 3. This value is based on complex performance considerations, and no direct interface is provided for changing it. The default hardware queue depth for non-IBM disk drives is 1. If a specific non-IBM disk drive does have the ability to buffer multiple requests, the system's description of that device should be changed accordingly.

As an example, the default characteristics of a non-IBM disk drive are displayed with the lsattr command, as follows:

# lsattr -D -c disk -s scsi -t osdisk
pvid          none Physical volume identifier      False
clr_q         no   Device CLEARS its Queue on error
q_err         yes  Use QERR bit
q_type        none Queuing TYPE
queue_depth   1    Queue DEPTH
reassign_to   120  REASSIGN time out value
rw_timeout    30   READ/WRITE time out value
start_timeout 60   START unit time out value

You can use SMIT (the fast path is smitty chgdsk) or the chdev command to change these parameters. For example, if your system contained a non-IBM SCSI disk drive hdisk5, the following command enables queuing for that device and sets its queue depth to 3:

# chdev -l hdisk5 -a q_type=simple -a queue_depth=3