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{{Short description|Type of computer file existing on IBM mainframe operating systems}}
{{about|
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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==Record format (RECFM)==
Regardless of organization, the physical structure of each record is essentially the same, and is uniform throughout the data set. This is specified in the DCB <code>RECFM</code> parameter. <code>RECFM=F</code> means that the records are of fixed length, specified via the <code>LRECL</code> parameter. <code>RECFM=V</code> specifies a variable-length record. V records when stored on media are prefixed by a Record Descriptor Word (RDW) containing the integer length of the record in bytes and flag bits. With <code>RECFM=FB</code> and <code>RECFM=VB</code>, multiple logical records are grouped together into a single [[Block (data storage)|physical block]] on tape or DASD. FB and VB are <
The <code>BLKSIZE</code> parameter specifies the maximum length of the block. <code>RECFM=FBS</code><ref>{{cite web
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|title=Example: Record format VBS
|website=[[IBM]]
|quote=Variable-length, blocked, spanned (VBS)}}</ref> could be also specified, meaning <
This mechanism eliminates the need for using any "[[delimiter]]" byte value to separate records. Thus data can be of any type, including binary integers, floating-point, or characters, without introducing a false end-of-record condition. The data set is an abstraction of a collection of records, in contrast to files as unstructured streams of bytes.
== Partitioned data set ==
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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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| pages = 107–110
| url = http://bitsavers.org/pdf/ibm/370/OS_VS2/Release_2_1973/GC26-3836-1_OS_VS_Access_Method_Services_May1974.pdf
|
| publisher = [[IBM]]
| access-date = May 19, 2022
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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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