Striping

[AIXUser53]

Hi Guys,

I have an SCSI raid card installed on my system with 4 72GB drives. I would like to created a disk array and do a raid 0.(I am doing raid 0 becuase this is a test server)
What is the Command Queue Depth should be set to. By default is 16, but how do I know that is the right setting.

Thanks in advance.

 

[MoshiachNow]

FRom IBM performance guide:

Setting SCSI-Adapter and Disk-Device Queue Limits
The operating system has the ability to enforce limits on the number of I/O requests that can be
outstanding from the SCSI adapter to a given SCSI bus or disk drive. These limits are intended to exploit
the hardware’s ability to handle multiple requests while ensuring that the seek-optimization algorithms in
the device drivers are able to operate effectively.
For non-IBM devices, it is sometimes appropriate to modify the default queue-limit values that have been
chosen to handle the worst possible case. The following sections describe situations in which the defaults
should be changed and the recommended new values.
Non-IBM Disk Drive
For IBM disk drives, the default number of requests that can be outstanding at any given time is 3 (8 for
SSA). 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
Non-IBM Disk Array
A disk array appears to the operating system as a single, rather large, disk drive. A non-IBM disk array,
like a non-IBM disk drive, is of class disk, subclass SCSI, type osdisk (which stands for ?Other SCSI Disk
Drive?). Because a disk array actually contains a number of physical disk drives, each of which can handle
multiple requests, 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 hdisk7 were an eight-disk non-IBM disk array,
an appropriate change would be as follows:
# chdev -l hdisk7 -a q_type=simple -a queue_depth=24
If the disk array is attached through a SCSI-2 Fast/Wide SCSI adapter bus, it may also be necessary to
change the outstanding-request limit for that bus.

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