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{{about|computer files|data communications|modem}}
In the context of [[IBM]] [[mainframe computer]]s in the [[
A data set is typically stored on a [[direct access storage device]] (DASD) or [[magnetic tape]],<ref>{{cite web
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|quote=... non-VSAM ...
|title=What is a generation data group? |website=IBM.com}}</ref> that are successive generations of historically-related data<ref name=G.sets>{{cite web |title=Generation data sets
|website=[[IBM]] |quote=successive, historically related, |url=https://www.ibm.com/support/knowledgecenter/en/SSLTBW_2.3.0/com.ibm.zos.v2r3.ieab500/iea3b5_Generation_data_sets_.htm}}</ref> stored on an IBM mainframe (running [[OS/360 and successors|OS/360 and its successors]] or [[DOS/
A GDG is usually cataloged.<ref name=G.sets/>
An individual member of the GDG collection is called a "''Generation Data Set''."<ref name=G.sets/><ref>"A generation data set is one of ...</ref> The latter may be identified by an absolute number, {{code|ACCTG.OURGDG(1234)}}, or a relative number: {{code|(-1)}} for the previous generation, {{code|(0)}} for the current one, and {{code|(+1)}} the next generation.<ref>{{cite web
|url=http://mainframewizard.com/content/what-gdg |title=What is a GDG?}}</ref>
A GDG specifies how many generations of a data set are to be kept and at what age a generation will be deleted. Whenever a new generation is created, the system checks whether one or more obsolete generations are to be deleted.
The purpose of GDGs is to automate archival, using the command language [[Job Control Language|JCL]], the data set name given is generic. When DSN appears, the GDG data set appears along with the history number, where
(0) is the most recent version
(-1), (-2), ... are previous generations
(+1) a new generation (see DD)
Another use of GDGs is to be able to address all generations simultaneously within a JCL script without having to know the number of currently available generations. To do this, you have to omit the parentheses and the generation number in the JCL when specifying the dataset.
===GDG JCL & features===
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| access-date = May 19, 2022
}}
</ref> of the {{pslink|Support programs for OS/360 and successors|IEHPROGM}} utility or the {{code|DEFINE GENERATIONGROUP}} statement<ref>{{cite manual
| title = OS/VS Access Method Services
| id = GC26-3836-1
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|title=IDCAMS – Create and delete GDG base using JCL
|url=http://code.xmlgadgets.com/2011/05/16/idcams-create-and-delete-gdg-base/comment-page-1}}</ref>
====Example====
Creation of a standard GDG for five safety scopes, each at least 35 days old:
<syntaxhighlight lang="jcl">
//STEP1 EXEC PGM=IDCAMS
//SYSPRINT DD SYSOUT=*
//SYSIN DD *
DEFINE GDG (NAME('DB2.FULLCOPY.DSNDB04.TSTEST') LIMIT(5) SCRATCH FOR(35))
/*
</syntaxhighlight>
Delete a standard GDG:
<syntaxhighlight lang="jcl">
//STEP3 EXEC PGM=IDCAMS
//SYSPRINT DD SYSOUT=*
//SYSIN DD *
DELETE DB2.FULLCOPY.DSNDB04.TSTEST GDG FORCE
/*
</syntaxhighlight>
==References==
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