Veritas Volume Manager 5.0.1 Administrator's Guide, HP-UX 11i v3, First Edition, November 2009

Table 16-1
Kernel tunable parameters for VxVM (continued)
DescriptionParameter
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 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 this tunable 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 the value of this tunable, 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_maxspecialio
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.
vol_subdisk_num
If set to 0, volcvm_smartsync disables SmartSync on
shared disk groups. If set to 1, this parameter enables the
use of SmartSync with shared disk groups.
See “SmartSync recovery accelerator” on page 61.
volcvm_smartsync
535Performance monitoring and tuning
Tuning VxVM