Virtual Storage Access Method: Difference between revisions

Revision as of 12:27, 5 July 2025

Revision as of 12:33, 5 July 2025

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===Sequential organization===

An '''entry-sequenced data set''' ('''ESDS''') is a type of VSAM.<ref name="demystified"/>{{rp|5}} Records are accessed based on their sequential order, that is, the order in which they were written to the file;<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=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|title=ABCs of z/OS System Programming Volume 3|citeseerx=10.1.1.469.8853}}</ref> which means that accessing a particular record involves searching all the records [[Sequential access|sequentially]] until it is located, or by using a relative physical address (''Relative byte address'', RBA), i.e. the number of bytes from the beginning of the file to start reading.<ref name="demystified"/>{{rp|28}}

{{further|entry-sequenced data set}}

}}</ref> Records are loaded irrespective of their contents and their byte addresses cannot be changed.

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=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

|title=ABCs of z/OS System Programming Volume 3|citeseerx=10.1.1.469.8853

}}</ref> Records are loaded irrespective of their contents and their byte addresses cannot be changed.

===Indexed organization===

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==Further reading==

*{{cite manual|title = z/OS 2.5 MVS Programming: Assembler Services Guide|id = SA23-1368-50|date = September 30, 2021|url = https://www.ibm.com/servers/resourcelink/svc00100.nsf/pages/zOSV2R5sa231368/$file/ieaa600_v2r5.pdf|ref = {{sfnref|ASMGUIDE}}| publisher = IBM}}

;ASMGUIDE

*{{cite web|url=http://www.mvsforums.com/techfaqsvsam.html|title=MVS Forums - Technical FAQ's - VSAM|website=Mvsforums.com|accessdate=12 December 2017}}

:{{cite manual

| title = z/OS 2.5 MVS Programming: Assembler Services Guide

| id = SA23-1368-50

| date = September 30, 2021

| url = https://www.ibm.com/servers/resourcelink/svc00100.nsf/pages/zOSV2R5sa231368/$file/ieaa600_v2r5.pdf

| ref = {{sfnref|ASMGUIDE}}

| publisher = IBM

}}

{{Mainframe I/O access methods}}