Editing Virtual Storage Access Method (section)

{{Short description|IBM disk file programming interface}}
{{merge from|Linear data set|Entry-sequenced data set|Key-sequenced data set|Relative record data set|discuss=Talk:Virtual_Storage_Access_Method|date=February 2025}}
'''Virtual Storage Access Method''' ('''VSAM''')<ref>{{cite magazine  |magazine=[[Datamation]]
   |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|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=https://www.redbooks.ibm.com/redbooks/pdfs/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=August 2022}}</ref> The KSDS, RRDS and ESDS organizations contain records, while the LDS organization (added later to VSAM) contains a sequence of pages with no intrinsic record structure, for use as a [[memory-mapped file]].