What is a VSAM?




What is a VSAM?

In 1970 when IBM launched the virtual storage operating systems OS/VS1 and OS/VS2, the access methods (ISAM and BDAM) used in the older versions of the operating systems when used with the newer OS started giving performance issues. To overcome performance issues and device dependence, new access methods were introduced replacing the older access methods. This new access method was termed as VSAM by IBM.


VSAM was introduced as a replacement for older access methods and was intended to add function, to be easier to use and to overcome problems of performance and device-dependence. VSAM was introduced in the 1970s when IBM announced virtual storage operating systems (DOS/VS, OS/VS1 and OS/VS2) for its new System/370 series, as successors of the DOS/360 and OS/360 operating systems running on its System/360 computer series. While backwards compatibility was maintained, the older access methods suffered from performance problems due to the address translation required for virtual storage.

VSAM datasets comprises of control interval and control areas.

; ; ; ; Control interval-

It has multiple logical records and is basic unit that takes part in transfers between the storage devices and processor. One control interval may be made up of one or more physical blocks on the storage device(DASD). The logical records in a control interval are stored one after the other. The unused or free space in a control interval is after the logical records. The free space is followed by control information which is at record level (RDF –Record Descriptor Field) and at control interval level (CIDF – Control Interval Descriptor Field). These control intervals are fixed size blocks and can be compared with blocks in sequential file. The size of CI is generally determined by the type of access method used. Control interval size for a dataset will always be same.


Control Areas-

It contain multiple control intervals. The size of CA is also determined by the type of access method to be used.


There are four types’ dataset organizations in VSAM.

  • Key Sequenced Data Set (KSDS)- Organization is Indexed.
  • Relative Record Data Set (RRDS) – Organization is relative
  • Entry Sequenced Data Set (ESDS) – Organization is sequential
  • Linear Data Set(LDS).