Veritas Volume Manager 5.0.1 Administrator's Guide, HP-UX 11i v3, First Edition, November 2009

This policy is designed to optimize the use of caching in disk drives
and RAID controllers. The size of the cache typically ranges from
120KB to 500KB or more, depending on the characteristics of the
particular hardware. During normal operation, the disks (or LUNs)
are logically divided into a number of regions (or partitions), and I/O
from/to a given region is sent on only one of the active paths. Should
that path fail, the workload is automatically redistributed across the
remaining paths.
You can use the size argument to the partitionsize attribute to specify
the partition size. The partition size in blocks is adjustable in powers
of 2 from 2 up to 231. A value that is not a power of 2 is silently
rounded down to the nearest acceptable value.
Specifying a partition size of 0 is equivalent to specifying the default
partition size.
The default value for the partition size is 1024 blocks (1MB). Specifying
a partition size of 0 is equivalent to the default partition size of 1024
blocks (1MB).
The default value can be changed by adjusting the value of the
dmp_pathswitch_blks_shift tunable parameter.
See “DMP tunable parameters ” on page 540.
Note: The benefit of this policy is lost if the value is set larger than
the cache size.
For example, the suggested partition size for an Hitachi HDS 9960
A/A array is from 16,384 to 65,536 blocks (16MB to 64MB) for an I/O
activity pattern that consists mostly of sequential reads or writes.
The next example sets the balanced I/O policy with a partition size of
2048 blocks (2MB) on the enclosure enc0:
# vxdmpadm setattr enclosure enc0 \
iopolicy=balanced partitionsize=2048
balanced
[partitionsize=size]
This policy sends I/O on paths that have the minimum number of
outstanding I/O requests in the queue for a LUN. No further
configuration is possible as DMP automatically determines the path
with the shortest queue.
The following example sets the I/O policy to minimumq for a JBOD:
# vxdmpadm setattr enclosure Disk \
iopolicy=minimumq
This is the default I/O policy for all arrays.
minimumq
177Administering Dynamic Multipathing
Administering DMP using vxdmpadm