VSAM

VSAM is a high-performance access method used in the MVS, OS/390 and VSE/ESA operating systems. It was initially released by IBM in 1973 and is part of the Base product.

VSAM provides a number of data set types or data organization schemes. They are:

• Key-sequenced data set (KSDS)
• Entry-sequenced data set (ESDS)
• Relative record data set (RRDS)
• Variable-length relative record data set (VRRDS)
• Linear data set (LDS)

Installations have been using VSAM data sets to hold more and more of their data to the point where many have reached the 4-gigabyte architectural limit for the size of VSAM data sets. Beginning with DFSMS V1.3, you can create and use VSAM KSDSs that can be much larger than the 4-gigabyte limit imposed on any VSAM data set defined before this release. DFSMS V1.5 allows non-KSDS file types (ESDS, RRDS, VRRDS and LDS) to exceed 4 gigabytes.

VSAM record-level sharing (RLS) was introduced to provide the value of the Parallel Sysplex to the existing applications. RLS itself does not provide transactional recovery. CICS provides a file access interface on top of VSAM. It is a CICS file control function that includes transactional recovery for VSAM files. This isolation and rollback capability enables VSAM data to be shared among CICS applications.