VSAM Extent Consolidation

The system consolidates adjacent extents for VSAM data sets when extending on the same volume. VSAM extent consolidation is automatic and requires no action on your part. If the extents are adjacent, the new extent is incorporated into the previous extent.

Example: The old extent begins on cylinder 6, track 0, and ends on cylinder 9, track 14, and the new extent begins on cylinder 10, track 0, and ends on cylinder 12, track 14. The two extents are combined into one extent beginning on cylinder 6, track 0, and ending on cylinder 12, track 14. Instead of two extents, there is only one extent. Because VSAM combines the two extents, it does not increment the extent count, which reduces the amount of extents.

Example: You allocate a VSAM data set with CYLINDERS(3 1). The data set initially gets three cylinders and an additional cylinder every time the data set is extended. Suppose you extend this data set five times. If none of the extents are adjacent, the LISTCAT output shows allocations of cylinders 3,1,1,1,1,1, or a total of eight cylinders.

Results: Depending on which extents are adjacent, the LISTCAT output might show allocations of cylinders 5,1,1,1, or cylinders 3,5, or cylinders 3,2,3, as follows:

For the 5,1,1,1 example, only the first three extents are adjacent.
For the 3,5 example, the first and second extent are not adjacent, but the third through eighth extent are adjacent.
For the 3,2,3 example, the first and second extent are not adjacent, the second and third extents are adjacent, the third and fourth extents are not adjacent, and the last three extents are adjacent.

All types of SMS-managed VSAM data sets (KSDS, ESDS, RRDS, VRRDS, and LDS) use extent consolidation.

Restriction: VSAM does not support extent consolidation for the following types of data sets:

Key-range data sets
System data sets such as page spaces
Catalogs
VVDSs
Non-system managed data sets
Imbedded or replicated indexes
VSAM data sets that you access using record-level sharing