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

Tuning VxVM
296 VERITAS Volume Manager Administrator’s Guide
vol_maxspecialio
The maximum size of an I/O request that can be issued by an ioctl call. Although
the ioctl request itself can be small, it can request a large I/O request be performed.
This tunable limits the size of these I/O requests. If necessary, a request that exceeds
this value can be failed, or the request can be broken up and performed
synchronously.
The default value for this tunable is 256 sectors (256KB).
Raising this limit can cause difficulties if the size of an I/O request causes the process
to take more memory or kernel virtual mapping space than exists and thus deadlock.
The maximum limit for vol_maxspecialio is 20% of the smaller of physical
memory or kernel virtual memory. It is inadvisable to go over this limit, because
deadlock is likely to occur.
If stripes are larger than vol_maxspecialio, full stripe I/O requests are broken up,
which prevents full-stripe read/writes. This throttles the volume I/O throughput for
sequential I/O or larger I/O requests.
This tunable limits the size of an I/O request at a higher level in VxVM than the level
of an individual disk. For example, for an 8 by 64KB stripe, a value of 256KB only
allows I/O requests that use half the disks in the stripe; thus, it cuts potential
throughput in half. If you have more columns or you have used a larger interleave
factor, then your relative performance is worse.
This tunable must be set, as a minimum, to the size of your largest stripe (RAID-0 or
RAID-5).
vol_subdisk_num
The maximum number of subdisks that can be attached to a single plex. There is no
theoretical limit to this number, but it has been limited to a default value of 4096. This
default can be changed, if required.
volcvm_smartsync
If set to 0, volcvm_smartsync disables SmartSyncon shared disk groups.If set to 1,
this parameter enables the use of SmartSync with shared disk groups. See“SmartSync
Recovery Accelerator” on page 49 for more information.