VERITAS Volume Manager 3.5 Administrator's Guide (September 2002)

Chapter 12, Performance Monitoring and Tuning
Tuning VxVM
293
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 valueof this parameteris set to10so that1024blocks (1MB)ofcontiguous
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.
The default for this tunable is 50 ticks.
Increasing this value results in slower recovery operations and consequently lower
system impact while recoveries are being performed.