VERITAS Volume Manager 4.1 Administrator's Guide

Performance Monitoring and Tuning
Tuning VxVM
Chapter 12 411
vol_maxparallelio
The number of I/O operations that the vxconfigd (1M) daemon is
permitted to request from the kernel in a single VOL_VOLDIO_READ per
VOL_VOLDIO_WRITE ioctl call.
The default value for this tunable is 256. It is not desirable to change this
value.
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.