Allocating Space

It is necessary to distinguish between two types of allocation under VSAM.

  1. The actual space allocation on the disk
  2. Allocation within the data set itself

Space for VSAM components must be allocated on the DASD using the DEFINE commands. You can only allocate space for the master catalog, a user catalog, a data space, and a UNIQUE cluster.

In defining the actual DASD space for components, there are parameters for the DEFINE SPACE command that allows the user to include a “secondary allocation” specification. These parameters are CYLINDERS, RECORDS, BLOCKS, and TRACKS. They have this secondary facility only as a syntactic compatibility with the OS/VS access method services commands. That is, VSE (and, therefore, CMS) does not perform secondary space allocation on a DASD.

The facility does exist under VSE (and CMS) to extend data or index components through already allocated data space, catalog extents, or UNIQUE cluster extents. Thus, the CYLINDERS, TRACKS, RECORDS, and BLOCKS parameters of the DEFINE commands for alternate indexes, clusters, and catalogs do not dynamically allocate DASD space but only extend a component through existing space.