Virtual Storage Access Method: Difference between revisions

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'''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 unathorizedunauthorized 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 ''[[Direct 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 &mdash; for example 4 [[kilobyte]]s &mdash; 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.

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=== Sequential VSAM organization ===

{{seefurther|Entry Sequenced Data Set}}

An ESDS may have an index defined to it to enable access via keys, by defining an Alternate Index.Records in ESDS are stored in order in which they are written by address access.Records are loaded irrespective of their contents and their byte addresses cannot be changed.

=== Indexed VSAM organization ===

{{seefurther|Key Sequenced Data Set}}

A KSDS has two parts: the index component and the data component. These may be stored on separate disk volumes.

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=== Relative VSAM organization ===

{{seefurther|Relative Record Data Set}}

An RRDS may have an index defined to it to enable access via keys, by defining an Alternate Index.

=== Linear VSAM organization ===

{{seefurther|Linear Data Set}}

An LDS is an unstructured VSAM dataset with a control interval size of a multiple of 4K. It is used by certain system services.

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* Local Shared Resources (LSR), is optimised for "random" or direct access. LSR access is easy to achieve from [[CICS]].

* Global Shared Resources (GSR)

* Non-Shared Resources (NSR), which is optimised for sequential access. NSR access has historically been easier to use than LSR for batch programs.

* Distributed File Management (DFM), an implementation of a [[Distributed Data Management Architecture]] server, enables programs on remote computers to create, manage, and access VSAM files.

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Linear data sets were added later, followed by VSAM RLS and then Transactional VSAM.

== Notes ==

{{Reflist|group=NB}}

== See also ==

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* [[Geneva ERS]]

* [[Record Management Services]], a similar system developed by [[Digital Equipment Corporation]]

== Notes ==

{{Reflist|group=NB}}

== References ==