VSAM Data Sets

VSAM data sets are formatted differently than non-VSAM data sets. Except for linear data sets, VSAM data sets are collections of records, grouped into control intervals. The control interval is a fixed area of storage space in which VSAM stores records. The control intervals are grouped into contiguous areas of storage called control areas. To access VSAM data sets, use the VSAM access method.

VSAM arranges records by an index key, by a relative byte address, or by a relative record number. VSAM data sets are cataloged for easy retrieval. The following are the different types of VSAM data sets:

Key-sequenced data set (KSDS): Contains records in order by a key field and can be accessed by the key or by a relative byte address. The key contains a unique value, such as an employee number or part number.
Entry-sequenced data set (ESDS): Contains records in the order in which they were entered and can only be accessed by relative byte address. An ESDS is similar to a sequential data set.
Relative-record data set (RRDS): Contains records in order by relative-record number and can only be accessed by this number. Relative records can be fixed-length or variable-length.
Linear data set (LDS): Contains data that can be accessed as byte-addressable strings in virtual storage. A linear data set does not have imbedded control information that other VSAM data sets hold. You also can access linear data sets by using the z/OS data in virtual (DIV) access method or window services. VSAM record-level sharing (RLS) is not available for VSAM linear data sets.

Related reading: For more information on VSAM data sets, see z/OS DFSMS Using Data Sets. For more information on DIV and window services, see z/OS MVS Programming: Assembler Services Guide.