topic Re: What are the maximum values for SCSI queue depth ? in Operating System - HP-UX

What are the maximum values for SCSI queue depth ?

CAS_2 — Mon, 17 Mar 2014 02:50:27 GMT

Hi

How can I get the maximum values for the scsi queue depth in HP-UX according to type of disk (internal SCSI disk, XP LUN, VA LUN, EVA LUN, ...) ?

Thanx.

P.S. This thread has been moved from Disk to HP-UX > sysadmin. -HP Forum Moderator

Re: What are the maximum values for SCSI queue depth ?

Peter Mattei — Tue, 05 Sep 2006 02:51:26 GMT

Hi

HP-UX default Q-depth value is 8 for a LUN.
You can change it for an individual device using
scsictl scsictl -m queue_depth=xx /dev/rdsk/cxtxdx

The max Q-depth per array port for the following arrays are:

XPxxxx 1024
VA7xxx 750
EVAxxxx 2048

I do not know the values for internal disks but assume them very low!

Cheers
Peter

Re: What are the maximum values for SCSI queue depth ?

sajeer_2 — Tue, 05 Sep 2006 05:59:55 GMT

Hi cas,

Maximum queue depth you can set with scsictl command is 256.

from man page of scsictl,

"For devices that support a queue depth greater than the system default, this mode controls how many I/Os the driver will ttempt to queue to the device at any one time. Valid values are (1-255). Some disk
devices will not support the maximum queue depth settable by this command.Setting the queue depth in software 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."

sajeer

Re: What are the maximum values for SCSI queue depth ?

CAS_2 — Tue, 05 Sep 2006 09:02:11 GMT

Yes, sajeer.

I know the maximum value accepted by scsictl command is 256. But, as shown in man pages,

"Some disk
devices will not support the maximum queue depth settable by this command.Setting the queue depth in software 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."

I'm interested in not setting a value larger than the disk can handle in order to prevent I/Os to be held off.
What is that value ?
I guess that value is different from an internal SCSI disk than a XP LUN.
Where can be found that value ?

Re: What are the maximum values for SCSI queue depth ?

Peter Mattei — Tue, 05 Sep 2006 11:10:33 GMT

Oh, this is hard to find and differs from OS to OS and by IO pattern!

However, performance tests for the XP have shown that it is best to keep the Q-depth for a single LUN below 17 and for an XP CHIP port below 222.

Cheers
Peter