Veritas Volume Manager 5.1 SP1 Administrator"s Guide (5900-1506, April 2011)

Table 15-1
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 1024 sectors (1MB).
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.
The default value is 1.
See “SmartSync recovery accelerator” on page 59.
volcvm_smartsync
499Performance monitoring and tuning
Tuning VxVM