Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

466 Performance monitoring and tuning
Tuning VxVM
vol_maxio
The maximum size of logical I/O operations that can be performed without breaking up
the request. I/O requests to VxVM that are larger than this value are broken up and
performed synchronously. Physical I/O requests are broken up based on the capabilities of
the disk device and are unaffected by changes to this maximum logical request limit.
The default value for this tunable is 256 sectors (256KB).
Note: The value of voliomem_maxpool_sz must be at least 10 times greater than the
value of vol_maxio.
If DRL sequential logging is configured, the value of voldrl_min_regionsz must be
set to at least half the value of vol_maxio.
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_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.