scsi_max_qdepth(5)scsi_max_qdepth(5)NAMEscsi_max_qdepth - maximum number of I/Os that target will queue up for
execution (OBSOLETE)
VALUES
Failsafe
Default
Allowed values
Recommended values
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.
DESCRIPTION
Note: This tunable is obsolete and is replaced by the attribute which
can be set through the 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
Queue depth is synonymous to the tagged queuing. When supported by a
target, it allows the target to accept multiple SCSI commands for exe‐
cution. 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 termi‐
nates any additional I/O and returns a status to the initiator. Tar‐
gets may support less than 256 commands to be queued, hence the factory
defaults to
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 adminis‐
trator can change the queue depth on a per device basis using 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?
Anyone.
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 queu‐
ing. Setting the queue depth to a value larger than the disk can han‐
dle will result in I/Os being held off once a condition exists on the
disk. A mechanism exists that will lower the queue depth of the device
in case of condition avoiding infinite conditions on that device. Nev‐
ertheless, this mechanism will periodically try higher queue depths and
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 perfor‐
mance when a lower queue depth value is set.
What Other Tunables Should Be Changed at the Same Time?
None.
WARNINGS
All HP-UX kernel tunable parameters are release specific. This parame‐
ter 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 val‐
ues. For information about the effects of installation on tunable val‐
ues, consult the documentation for the kernel software being installed.
For information about optional kernel software that was factory
installed on your system, see at
AUTHOR
was developed by HP.
SEE ALSOscsictl(1M), ioctl(2), scsi(7).
OBSOLETE Tunable Kernel Parameters scsi_max_qdepth(5)
