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481Performance monitoring and tuning
Tuning VxVM
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.
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.