'''Virtual Storage Access Method''' ('''VSAM''') is an [[IBM]] [[Direct access storage device|DASD]] file storage [[access method]], first used in the [[OS/VS1]], OS/VS2 [[OS/VS2 (SVS)|Release 1]] (SVS) and [[MVS|Release 2]] (MVS) operating systems, later used throughout the [[Multiple Virtual Storage]] (MVS) architecture and now in [[z/OS]]. Originally a [[record-oriented filesystem]].,{{#tag:ref|With the exception of catalogs, page spaces and swap<ref group=NB>No longer used.</ref> spaces, which unauthorized applications could access only via specialized OS services. Not to mention the fact that it's been in VSE for ever too and is used in z/VSE|group="NB"|name=notfile}}, VSAM comprises four<ref group=NB name=notfile/> [[data set (IBM mainframe)|data set]] ''organizations'': [[Key Sequenced Data Set]] (KSDS), [[Relative Record Data Set]] (RRDS), [[Entry Sequenced Data Set]] (ESDS) and [[Linear Data Set]] (LDS). 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]].
IBM uses the term ''data set'' in official documentation as a synonym of ''file'', and ''[[Directdirect access storage device]]'' (''DASD'') because it supported other devices similar to ''disk drives''.
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 —{{snd}} for example 4 [[kilobyte]]s —{{snd}} while Control Area sizes are measured in disk tracks or cylinders. Control Intervals are the units of transfer between disk and computer so a read request will read one complete Control Interval. Control Areas are the units of allocation so, when a VSAM data set is defined, an integral number of Control Areas will be allocated.
The Access Method Services utility program [[IBM mainframe utility programs#IDCAMS|IDCAMS]] is commonly used to manipulate ("delete and define") VSAM data sets.
