Virtual storage access method
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Virtual Storage Access Method (VSAM) is an IBM disk file storage scheme first used in the OS/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 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 structures.
[edit] References
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