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Oracle Tips by Burleson |
Disk I/O Tuning
Chapter 3:
HP-UX SCSI Tuning
This
value should only be raised in 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.
Remember, 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 similar to
the process that lowers it to 1 on SUN Solaris. Nevertheless, this
mechanism will periodically try higher queue depths and QUEUE FULL
conditions will arise.
This
parameter should be lowered when the connected SCSI devices support
smaller queue depth or for load balancing. However, devices that
support higher queue depth may not deliver optimal performance when
a lower queue depth value is set.
The other tunable
parameter in HP-UX for SCSI is scsi_maxphys, which sets the maximum
allowed length of an IO on all SCSI devices. scsi_maxphys has a
minimum and maximum setting of 1048576 (one megabyte) except on
V-class systems, where it has the range of values:
16777215
and
33554432.
The above text is
an excerpt from:
Oracle Disk I/O Tuning
Disk IO Performance & Optimization for Oracle
Databases
ISBN
0-9745993-4-4
by Mike Ault
Oracle Consulting
Services
Burleson
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