scsi_max_qdepth(5)						File Formats Manual						scsi_max_qdepth(5)

NAME

       scsi_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       Note: This tunable is obsolete and is replaced by the attribute which can be set through the command.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
       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 status to the initiator.
       Targets 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  administrator	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 queuing.  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.  Nevertheless, 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 performance 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 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 tun-
       able values, 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 ALSO

       scsictl(1M), ioctl(2), scsi(7).

OBSOLETE
						     Tunable Kernel Parameters						scsi_max_qdepth(5)