Virtual Storage Access Method: Difference between revisions

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Revision as of 06:20, 7 April 2021

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|url=https://www.datamation.com/netsys/article.php/3677001/New-Life-for-Legacy-Systems-at-LaBarge.htm

|title=New Life for Legacy Systems at LaBarge

|date=May 11, 2007}}</ref> 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|key-sequenced]] (KSDS), [[Relative Record Data Set|relative record]] (RRDS), [[Entry Sequenced Data Set|entry-sequenced]] (ESDS) and [[Linear Data Set|linear]] (LDS).<ref name="demystified">{{cite book|url=http://www.redbooks.ibm.com/pubs/pdfs/redbooks/sg246105.pdf |series=Redbooks |title=VSAM Demystified |first1=Mary |last1=Lovelace |first2=Jose |last2=Dovidauskas |first3=Alvaro |last3=Salla |first4=Valeria |last4=Sokal |pages=5 |chapter=1.3.2 Record management |edition=3 |publisher=IBM |date=March 2013}}</ref> 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]].

==Overview==

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The Access Method Services utility program [[IBM mainframe utility programs#IDCAMS|IDCAMS]] is commonly used to manipulate ("delete and define") VSAM data sets. Custom programs can access VSAM datasets through Data Definition (DD) statements in [[Job Control Language]] (JCL), via dynamic allocation or in online regions such as in [[CICS|Customer Information Control System]] (CICS).

Both [[IMS/DB]]{{fact}} and [[IBM DB2|DB2]]<ref are implemented on top of VSAM and use its underlying [[data structure]]s.name="demystified"/>{{rp|41}}<ref>{{cite web

|url=http://IBMmainframes.com/about44870.html

|title=Underlying VSAM file of a DB2 table}}</ref><ref>{{cite web

|url=https://www-01.ibm.com/servers/resourcelink/svc00100.nsf/pages/izodav110sc279034/%24file/azk1b100.pdf

|title=User's Guide}}</ref><ref>{{cite webare implemented on top of VSAM and use its underlying [[data structure]]s.

|url=http://www.cedix.de/VorlesMirror/Band1/VSAM01.pdf |title=VSAM Demystified}}</ref>

==VSAM files==