VERITAS Volume Manager 4.1 Administrator's Guide

Performance Monitoring and Tuning
Tuning VxVM
Chapter 12408
The values of system tunables can be examined by selecting Kernel
Configuration > Configuration Parameters in the System
Administration Manager (SAM).
Tunable Parameters
The following sections describe specific tunable parameters.
dmp_pathswitch_blks_shift
The number of contiguous I/O blocks (expressed as an integer power of 2)
that are sent along a DMP path to an Active/Active array before
switching to the next available path.
The default value of this parameter is set to 10 so that 1024 blocks (1MB)
of contiguous I/O are sent over a DMP path before switching. For
intelligent disk arrays with internal data caches, better throughput may
be obtained by increasing the value of this tunable. For example, for the
HDS 9960 A/A array, the optimal value is between 15 and 17 for an I/O
activity pattern that consists mostly of sequential reads or writes.
The DMP path that is used for a block is calculated as follows:
path = (block# >> dmp_pathswitch_blks_shift) % number of
paths
vol_checkpt_default
The interval at which utilities performing recoveries or
resynchronization operations load the current offset into the kernel as a
checkpoint. A system failure during such operations does not require a
full recovery, but can continue from the last reached checkpoint.
The default value of the checkpoint is 10240 sectors (10MB).
Increasing this size reduces the overhead of checkpointing on recovery
operations at the expense of additional recovery following a system
failure during a recovery.
vol_default_iodelay
The count in clock ticks for which utilities pause if they have been
directed to reduce the frequency of issuing I/O requests, but have not
been given a specific delay time. This tunable is used by utilities
performing operations such as resynchronizing mirrors or rebuilding
RAID-5 columns.