VERITAS Volume Manager 3.5 Administrator's Guide (September 2002)

Chapter 12, Performance Monitoring and Tuning
Tuning VxVM
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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.
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.