vol_maxio and vol_maxspecialio are tunables in the Volume Manager (VxVM) component. In Storage Foundation 6.1, the tunables can be displayed & changed using the vxtune utility:

https://sort.Veritas.com/public/documents/sfha/6.1/linux/manualpages/html/man/volume_manager/html/man1m/vxtune.1m.html

https://sort.Veritas.com/public/documents/sfha/6.1/solaris/manualpages/html/man/volume_manager/html/man1m/vxtune.1m.html

https://sort.Veritas.com/public/documents/sfha/6.1/solaris/manualpages/html/man/volume_manager/html/man1m/vxtune.1m.html

vol_maxio

The maximum size of logical I/O operations that can be performed without breaking up the requests. This tunable controls the maximum size of a regular i/o without breaking up the i/o request. This relates to i/o's issued from applications and userland programs. In 6.1, vol_maxio defaults to 2048 sectors (1MB)

vol_maxspecialio 

The maximum size of an I/O request that can be set by an ioctl call. This tuneable controls the breakup of special I/O requests initiated from VxVM internally, such as atomic copies using ioctl() interface. In 6.1, vol_maxspecialio defaults to 4096 sectors (2MB)

Changes to these tuneables will take effect at boot time.

Refer to the Storage Foundation Administrator's guide for further information

https://sort.Veritas.com/documents

Note:

Operating Systems also have tunables which control the breakup of an i/o at the disk driver layer. i.e maxphys on Solaris, max_sectors_kb on Linux