Editing Virtual Storage Access Method (section)

==Overview==
An IBM ''Redbook'' named "VSAM PRIMER" (especially when used with the "Virtual Storage Access Method (VSAM) Options for Advanced Applications" manual) explains the concepts needed to make use of VSAM.<ref>{{cite web
   |url=http://www.computinghistory.org.uk/det/9393/IBM-International-Systems-Centers-VSAM-Primer-for-the-Intergrated-Catalog-Facility-in-an-MVS-Environment
   |title=VSAM Primer}}</ref> IBM uses the term ''data set'' in official documentation as a synonym for ''file'', and ''[[direct-access storage device]]'' (''DASD'') for devices with [[random access]] to data locations, such as disk drives, as opposed to devices such as tape drives that can only be read sequentially.

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.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).

Both [[IMS/DB]]{{fact|date=April 2021}} and [[IBM Db2|Db2]]<ref name="demystified"/>{{rp|41}}<ref>{{cite web
|url=https://www-01.ibm.com/servers/resourcelink/svc00100.nsf/pages/izodav110sc279034/%24file/azk1b100.pdf
|title=User's Guide|date=16 February 2024 }}</ref> are implemented on top of VSAM and use its underlying [[data structure]]s.