When using VxFS with Veritas Volume Manager (VxVM), VxVM by default breaks up I/O requests larger than 256K. When using striping, to optimize performance, the file system issues I/O requests that are up to a full stripe in size. If the stripe size is larger than 256K, those requests are broken up.
To avoid undesirable I/O breakup, you can increase the maximum I/O size by changing the value of the
vol_maxio parameter controls the maximum size of logical I/O operations that can be performed without breaking up a request. Logical I/O requests 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 the
vol_maxio logical request limit.
vol_maxio limit can cause problems if the size of an I/O requires more memory or kernel mapping space than exists. The recommended maximum for
vol_maxio is 20% of the smaller of physical memory or kernel virtual memory. It is not advisable to go over this limit. Within this limit, you can generally obtain the best results by setting
vol_maxio to the size of your largest stripe. This applies to both RAID-0 striping and RAID-5 striping.
To change or increase
vol_maxio, use the VxVM menus under SMIT. In the Volume Manager SMIT screens see this tunable listed as "Maximum I/O size":
Maximum I/O size (sectors) 
This value is in 512-byte sectors. The default value for this tunable is 512 sectors.
See the Veritas Volume Manager Administrator's Guide.