VERITAS Volume Manager 3.1 Administrator's Guide

Performance Monitoring
Tuning the Volume Manager
Chapter 9 403
vol_subdisk_num
This tunable is used to control the maximum number of subdisks that
can be attached to a single plex. There is no theoretical limit to this
number, but for practical purposes it has been limited to a default value
of 4096. This default can be changed if required.
vol_maxioctl
This value controls the maximum size of data that can be passed into the
Volume Manager via an ioctl call. Increasing this limit will allow larger
operations to be performed. Decreasing the limit is not generally
recommended since some utilities depend upon performing operations of
a certain size and may fail unexpectedly if they issue oversized ioctl
requests.
The default value for this tunable is 32768 bytes (32K).
vol_maxspecialio
This tunable controls the maximum size of an I/O that can be issued by
an ioctl call. The ioctl request itself may be small, but may have
requested a large I/O to be performed. This tunable limits the size of
these I/Os. If necessary, a request that exceeds this value may be failed,
or the I/O may be broken up and performed synchronously.
The default value for this tunable is 512 sectors (256K).
vol_maxio
This value controls the maximum size of logical I/O operations that can
be performed without breaking up the request. I/O requests to the
volume manager larger than this value are broken up and performed
synchronously. Physical I/Os 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 512 sectors (256K).
Raising this limit can cause difficulties if the size of an I/O causes the
process to take more memory or kernel virtual mapping space than
exists and thus deadlock. The maximum limit for vol_maxio is 20% of
the smaller of physical memory or kernel virtual memory. It is
inadvisable to go over this limit since deadlock is likely to occur.
If you have stripes larger than vol_maxio, full stripe I/Os are broken up