Virtual Storage Access Method (VSAM) is an IBM disk file storage scheme first used in the S/370 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 thence 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.
Both IMS/DB and DB2 are implemented on top of VSAM and use its underlying data structures.