scsi_max_qdepth.5 (2010 09)

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scsi_max_qdepth(5)
OBSOLETE scsi_max_qdepth(5)
(Tunable Kernel Parameters)
NAME
scsi_max_qdepth - maximum number of I/Os that target will queue up for execution (OBSOLETE)
VALUES
Failsafe
1
Default
8
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.
DESCRIPTION
Note: This tunable is obsolete and is replaced by the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 com-
mand outstanding at any point in time. The number of commands that can be outstanding varies by dev-
ice, and is not known to HP-UX. To avoid overflowing this queue, HP-UX will not send more than a cer-
tain 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 adminis-
trator 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?
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 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.
HP-UX 11i Version 3: September 2010 − 1 − Hewlett-Packard Company 1