Jump to content United States-English
HP.com Home Products and Services Support and Drivers Solutions How to Buy
» Contact HP
More options
HP.com home
HP-UX Reference > S


Tunable Kernel Parameters
HP-UX 11i Version 3: February 2007

Technical documentation

» Feedback
Content starts here

 » Table of Contents

 » Index


scsi_max_qdepth — maximum number of I/Os that target will queue up for execution (OBSOLETE)






Allowed values

1 - 255

Recommended values

1 - 255

Most SCSI-2 and above devices accept multiple commands and have enough internal memory to support the default queue depth set by HP. You may change the default value to tune devices for higher throughput or load balancing.


Note: This tunable is obsolete and is replaced by the max_q_depth attribute which can be set through the scsimgr command. See scsimgr(1M).

Some SCSI devices support tagged queuing, which means that they can have more than one SCSI command outstanding at any point in time. The number of commands that can be outstanding varies by device, and is not known to HP-UX. To avoid overflowing this queue, HP-UX will not send more than a certain number of outstanding commands to any SCSI device. This tunable sets the default value for that limit. The default value can be overridden for specific devices using ioctl.

Queue depth is synonymous to the tagged queuing. When supported by a target, it allows the target to accept multiple SCSI commands for execution. Some targets can allow up to 256 commands to be stored from different initiators. This mechanism can help optimization for better performance. Once the target command queue is full, the target terminates any additional I/O and returns a QUEUE FULL status to the initiator. Targets may support less than 256 commands to be queued, hence the factory defaults to 8.

If the system has a combination of devices that support small and larger queue depths, then a queue depth can be set to a value which would work for most devices. For specific devices, the system administrator can change the queue depth on a per device basis using SIO_SET_LUN_LIMIT ioctl(). See scsictl(1M) for more on how to use ioctl().

The values for both 32-bit and 64-bit kernel are the same.

Who Is Expected to Change This Tunable?


Restrictions on Changing

Changes to this tunable take effect immediately.

When Should the Value of This Tunable Be Raised?

SCSI devices that have enough memory to support higher queue depth than the default set by HP. Such devices may offer better performance if the queue depth is set to a higher value.

What Are the Side Effects of Raising the Value of This Tunable?

The queue depth applies to all the SCSI devices that support tag queuing. Setting the queue depth 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. A mechanism exists that will lower the queue depth of the device in case of QUEUE FULL condition avoiding infinite QUEUE FULL conditions on that device. Nevertheless, this mechanism will periodically try higher queue depths and QUEUE FULL conditions will arise.

When Should the Value of This Tunable Be Lowered?

When the connected SCSI devices support smaller queue depth or for load balancing.

What Are the Side Effects of Lowering the Value of This Tunable?

Devices that support higher queue depth may not deliver optimal performance when a lower queue depth value is set.

What Other Tunables Should Be Changed at the Same Time?



All HP-UX kernel tunable parameters are release specific. This parameter has been obsoleted for HP-UX 11i Version 3.

Installation of optional kernel software, from HP or other vendors, may cause changes to tunable parameter values. After installation, some tunable parameters may no longer be at the default or recommended values. For information about the effects of installation on tunable values, consult the documentation for the kernel software being installed. For information about optional kernel software that was factory installed on your system, see HP-UX Release Notes at http://docs.hp.com.


scsi_max_qdepth was developed by HP.

Printable version
Privacy statement Using this site means you accept its terms Feedback to webmaster
© 1983-2007 Hewlett-Packard Development Company, L.P.