Tunable Kernel Parameters
NAME [Toc] [Back]
scsi_max_qdepth - maximum number of I/Os that target will queue up for
VALUES [Toc] [Back]
Default [Toc] [Back]
Allowed values [Toc] [Back]
1 - 255
Recommended values [Toc] [Back]
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
DESCRIPTION [Toc] [Back]
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
The values for both 32-bit and 64-bit kernel are the same.
Who Is Expected to Change This Tunable?
Hewlett-Packard Company - 1 - HP-UX 11i Version 2: Sep 2004
Tunable Kernel Parameters
Restrictions on Changing [Toc] [Back]
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
When Should the Value of This Tunable Be Lowered?
When the connected SCSI devices support smaller queue depth or for
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?
WARNINGS [Toc] [Back]
All HP-UX kernel tunable parameters are release specific. This
parameter may be removed or have its meaning changed in future
releases of HP-UX.
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
AUTHOR [Toc] [Back]
scsi_max_qdepth was developed by HP.
SEE ALSO [Toc] [Back]
scsictl(1M), ioctl(2), scsi(7).
Hewlett-Packard Company - 2 - HP-UX 11i Version 2: Sep 2004 [ Back ]