Virtual I/O for temporary data sets

Temporary data sets can be handled by a function called virtual I/O (VIO). Data sets for which VIO is specified are located in external page storage. However, to the access methods, the data sets appear to reside on real direct access storage devices. A VIO data set must be a non-VSAM data set; it can be sequential, partitioned, or direct, but not a PDSE or extended-format. VIO simulates a real device and provides the following advantages:
  • Elimination of some of the usual I/O device allocation and data management overhead for temporary DASD data sets.
  • Generally more efficient use of direct access storage space.

A VIO data set appears to the application program to occupy one unshared virtual (simulated) direct access storage volume. This simulated volume is like a real direct access storage volume except for the number of tracks and cylinders. A VIO data set can occupy up to 65 535 tracks even if the device being simulated does not have that many tracks.

A VIO data set always occupies a single extent (area) on the simulated device. The size of the extent is equal to the primary space amount plus 15 times the secondary amount (VIO data size = primary space + (15 × secondary space)). An easy way to specify the largest possible VIO data set in JCL is SPACE=(TRK,65535). You can set this limit lower. Specifying ALX (all extents) or MXIG (maximum contiguous extents) on the SPACE parameter results in the largest extent allowed on the simulated device, which can be less than 65 535 tracks.

There is no performance or resource penalty for overestimating how much space you need unless your system's accounting functions charge for it.

Do not allocate a VIO data set with zero space. Failure to allocate space to a VIO data set will cause unpredictable results when reading or writing.

A summary of the effects of ALX or MXIG with VIO data sets follows.

Simulated IBM Device
Number of Cylinders
3380
885
3390
1113
9345
1440