Virtual Storage Access Method: Difference between revisions

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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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|title=VSAM Primer}}</ref> 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 Interval]]s (CIs),<ref name="VSAM.CI1">{{cite web |title=VSAM – Components |url=https://www.tutorialZpoint.com/vsam/vsam_components.htm}}</ref><ref name=VSAM.CI2>{{cite web

|url=https://www.tutorialZpoint.com/vsam/vsam_components.htm

|title=VSAM - Components}}</ref><ref name=VSAM.CI2>{{cite web

|url=https://www.ibm.com/support/knowledgecenter/en/SSLTBW_2.1.0/com.ibm.zos.v2r1.idad400/d4221.htm

|title=Control Interval Size Limitations|website=[[IBM]] |date=27 March 2014 }}</ref> 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. 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).

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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.<ref>{{cite web

|url=https://walton.uark.edu/enterprise/IBM/systemZ/downloads/module3/VSAM-Introductory.ppt

|title=VSAM: introductory}}</ref><ref>{{cite web |title=Server Functionality |url=https://supportline.microfocus.com/documentation/books/tps62/mafunc.htm |quote=Sequential (VSAM ESDS – Entry Sequenced Dataset)}}</ref><ref>{{cite web

|url=https://supportline.microfocus.com/documentation/books/tps62/mafunc.htm

|title=Server Functionality

|quote=Sequential (VSAM ESDS - Entry Sequenced Dataset)}}</ref><ref>{{cite web

|url=http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.469.8853&rep=rep1&type=pdf

|quote=An ESDS VSAM data set contains records in the order in which they were entered

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* [[Geneva ERS]]

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

* [[Enscribe]], a similar system developed by [[Tandem Computers]]

== Notes ==

{{Reflist|group=NB}}