Virtual Sequential Access Method (VSAM) is an IBM disk file storage scheme first used in the S/370 and virtual storage. VSAM comprises three access methods: Keyed Sequential Data Set (KSDS), Relative Record Data Set (RRDS) and Entry Sequenced Data Set (ESDS).
KSDS has indexes and data split into CI (Control Interval) in CA (Control Area) and multi index levelled. Forward and backward compression is applied to key values.
Both IMS/DB and DB2 are implemented on top of VSAM and use its underlying data structures.
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