Veritas Volume Manager 5.1 SP1 Administrator"s Guide (5900-1506, April 2011)

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 504.
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]
Administering Dynamic Multi-Pathing
Administering DMP using vxdmpadm
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