Virtual Storage 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). See record-oriented filesystem.
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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This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.