Veritas Volume Manager 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

Tuning VxVM
414 VERITAS Volume Manager Administrator’s Guide
vol_maxioctl
The maximum size of data that can be passed into VxVM via an ioctl call. Increasing
this limit allows larger operations to be performed. Decreasing the limit is not generally
recommended, because some utilities depend upon performing operations of a certain
size and can fail unexpectedly if they issue oversized ioctl requests.
The default value for this tunable is 32768 bytes (32KB).
vol_maxkiocount
The maximum number of I/O operations that can be performed by VxVM in parallel.
Additional I/O requests that attempt to use a volume device are queued until the current
activity count drops below this value.
The default value for this tunable is 2048.
Because most process threads can only issue a single I/O request at a time, reaching the
limit of active I/O requests in the kernel requires 2048 I/O operations to be performed in
parallel. Raising this limit is unlikely to provide much benefit except on the largest of
systems.
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.