Virtual Storage Access Method (VSAM) is a file access method for the MVS operating system(z/ OS) for IBM mainframes. VSAM supports the management of different data types and has complex functions for accessing hard disks. It uses its own disk recording format and is therefore incompatible with other file access methods.
VSAM recognizes four different data sets, which differ mainly in the way they are recorded and accessed:
- Entry Sequenced Data Set (ESDS) In the ESDS data set, data is recorded and accessed sequentially. This type of data recording is used in Information Management System( ISM), z/OS formerly MVS and Unix.
- Key Sequenced Data Set (KSDS) The KSDS record is the most commonly used. In this record, the data set has a variable length and each record has one or more key fields into which records can be inserted or from which they can be extracted.
- Linear Data Set (LDS).. The LDS record is a byte stream used in MVS (z/OS) files. The LDS format is hardly used for application programs.
- Relative Record Data Set (RRDS) The RRDS format allows data to be recovered by the numerical order of records.