Virtual Storage Access Method: Difference between revisions

Revision as of 10:32, 8 May 2006

Revision as of 20:39, 9 May 2006

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'''Virtual Storage Access Method''' ('''VSAM''') is an [[IBM]] disk file storage scheme first used in the [[SOS/370]]VS2 operating system and later used throughout the [[MVS]] architecture. VSAM comprises four access methods: [[Key Sequenced Data Set]] (KSDS), [[Relative Record Data Set]] (RRDS), [[Entry Sequenced Data Set]] (ESDS) and [[Linear Data Set]] (LDS). See [[record-oriented filesystem]].

VSAM records can be of fixed or variable length. They are organised in fixed-size blocks called Control Intervals (CIs), and then into larger divisions called Control Areas (CAs). Control Interval sizes are measured in bytes — e.g. 4K — while Control Area sizes are measured in disk tracks or cylinders.

The program IDCAMS is commonly used to manipulate ("delete and define") VSAM [[Dataset|datasets]]. Custom programs can access VSAM datasets through data definitions (DD's) in Job Control Language ([[JCL]]) or in online regions such as CICS (Customer Information Control Systems.)

Both [[IMS/DB]] and [[DB2]] are implemented on top of VSAM and use its underlying [[Data_structure|data structures]].