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Virtual I/O for Temporary Data Sets z/OS DFSMS Using Data Sets SC23-6855-00 |
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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:
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.
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Copyright IBM Corporation 1990, 2014
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