Queue-depth recommendations

[rescuehp17]

By default the queue_depth value on HPUX is 8, and I am planning on changing to 16. What is the recommendation and is there a limit to add number of luns to the volume group when we change to 16. Please suggest!

[Bill Hassell · Accepted solution]

The number of luns is not limited by the queue_depth. For version 1.0 LVM layout, 255 luns (PVs) can be specified. For HP-UX 11.31, version 2 is available. For 2.0 and 2.1, the limit is 511 and for 2.2, there can be 2047 luns. Here are the limits:

http://h20565.www2.hpe.com/hpsc/doc/public/display?sp4ts.oid=4296010&docLocale=en_US&docId=emr_na-c02019992

The queue depth is below the LVM layer. There are no simple rules to estimate performance benefits. To start with, the simplest setup (one CPU, one path to JBOD disks and one program) will see no benefit since no queue can be generated. Every disk I/O must return to the program before the next I/O can be processed.

But for a multipath storage array, multiple CPUs and dozens of programs initiating I/O in parallel, the queue depth can be used to improve performance but only to a point. A slow processor speed, and/or slow disk array speeds will hit internal limits for handling large queue depths. For new CPUs such as the Integrity i2, i4 and i6 plus a Smart Array controller, qdepths might be set to 128 or higher as long as you have multiple paths available to your storage.  If your (unspecified) OS version can load balance, higher queue depths can improve performance. The best measurement tools are sar and Glance plus a repeatable work load to evaluate qdepth settings. Note that fine tuning disk array setups will also provide some help.

[rescuehp17]

Thank you very much! It really helped, Bill