VSAM (Virtual Storage Access Method ) is an access method for IBM's mainframe operating system, MVS, now called z/OS. VSAM succeeded and expanded upon an earlier IBM file access method, ISAM (Indexed Sequential Access Method).
Using VSAM, an enterprise can organize records in a file in physical sequence (the sequential order that they were entered), logical sequence using a key (for example, the employee ID number), or by the relative record number on direct access storage devices (DASD).
There are four types of VSAM data sets:
- Entry Sequenced Data Set (ESDS)
- Key Sequenced Data Set (KSDS)
- Linear Data Set (LDS)
- Relative Record Data Set (RRDS)
VSAM records can be of fixed or variable length. Many corporations run both old and new applications on mainframes that access VSAM files (also called data sets).
IBM now promotes DB2, a relational database management system, although VSAM linear datasets are still used to contain tablespaces and indexspaces within the system. Keyed sequence VSAM is also used by DB2 for the Boot Strap Dataset (BSDS).