In the course of an email exchange there was discussion about the performance of z/OS where the DASD was involved in an active-active environment – so every write I/O is mirrored over a network. There was a discussion about avoiding disk I/O for work files. VIO refers to data set allocations that exist in paging storage only. z/OS does not use a real device unless z/OS must page out the data set. If course you need enough real storage so you do not page!
In ISMF you can define a storage group, type of VIO which uses Virtual I/O.
I have a Storage Group of SGVIO which says use VIO if the data set size is less than 2000000 KB. If more than this is needed it will use DASD.
If you are in a mirrored environment, and you have DASD volumes which are just used for temporary files, or paging then these volumes do not need to be mirrored. (But you may want to mirror them in case some one puts a non temporary data set on the volume).
ColinPaice 