Virtual Storage Access Method (
VSAM)
[1] is an
IBM DASD file storage
access method, first used in the
OS/VS1, OS/VS2
Release 1 (SVS) and
Release 2 (MVS) operating systems, later used throughout the
Multiple Virtual Storage (MVS) architecture and now in
z/OS. Originally a
record-oriented filesystem,
[NB 2] VSAM comprises four
[NB 2] data set organizations:
key-sequenced (KSDS),
relative record (RRDS),
entry-sequenced (ESDS) and
linear (LDS).
[2] The KSDS, RRDS and ESDS organizations contain records, while the LDS organization (added later to VSAM) simply contains a sequence of pages with no intrinsic record structure, for use as a
memory-mapped file.