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VSAM vs non-VSAM

SPACE information is stored in different locations depending upon whether the data set or cluster being created is VSAM or non-VSAM.

SPACE information for VSAM clusters is held in the VSAM Volume Data Set (VVDS) with each volume having its own such data set.

This allows up to 127 extent allocations per volume, many more than for a non-VSAM data sets, which are limited to 16 per volume.

Where is SPACE information stored?

With non-VSAM data sets, the file label, known as the Data Set Control Block (DSCB) is held in the disk volume's Volume Table of Contents (VTOC). The principal DSCB is the Format 1 which can hold the locations of 3 areas of disk space, known as extents, i.e:

If a data set uses more than 3 locations of disk space, then a Format 3 DSCB is created to hold a maximum of 13 more extents, giving a total of 16, as illustrated here.

If any given request for SPACE was satisfied by the allocation of a single area of disk then there would be no confusion between requests and extents, but that is not the way the Storage Management routines work.

Let us look at some examples.