scsi_max_qdepth(5)                                       scsi_max_qdepth(5)
                          Tunable Kernel Parameters



 NAME    [Toc]    [Back]
      scsi_max_qdepth - maximum number of I/Os that target will queue up for
      execution

 VALUES    [Toc]    [Back]
    Failsafe
      1

    Default    [Toc]    [Back]
      8

    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
      balancing.

 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
      ioctl().

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

    Who Is Expected to Change This Tunable?
      Anyone.



 Hewlett-Packard Company            - 1 -      HP-UX 11i Version 2: Sep 2004






 scsi_max_qdepth(5)                                       scsi_max_qdepth(5)
                          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
      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?
      None.

 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
      http://docs.hp.com.

 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