Data set (IBM mainframe): Difference between revisions

Revision as of 18:02, 26 April 2019

Revision as of 17:57, 3 October 2019

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|title=What is a data set? |quote=data set .. a file that contains one or more records.}}</ref> and block structures determined by the <code>DSORG</code> (data set organization), <code>RECFM</code> (record format), and other parameters. These parameters are specified at the time of the data set allocation (creation), for example with [[Job Control Language]] <code>DD</code> statements. Within a running program they are stored in the [[Data Control Block]] (DCB), which is a data structure used to access data sets, for example using [[access method]]s.

Records in a datasetdata set may be fixed, variable, or “undefined” length.<ref>{{cite web

|url=https://www.ibm.com/support/knowledgecenter/zosbasics/com.ibm.zos.zconcepts/zconcepts_159.htm

|title=Data set record formats

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moves all members to the front of the data space and leaves free usable space at the back. (Note that in modern parlance, this kind of operation might be called [[defragmentation]] or [[garbage collection (computer science)|garbage collection]]; [[data compression]] nowadays refers to a different, more complicated concept.) PDS files can only reside on [[direct access storage device|DASD]], not on [[magnetic tape]], in order to use the directory structure to access individual members. Partitioned datasetsdata sets are most often used for storing multiple [[job control language]] files, [[IBM mainframe utility programs|utility]] control statements, and executable modules.

An improvement of this scheme is a Partitioned Data Set Extended (PDSE or PDS/E, sometimes just ''libraries'') introduced with DFSMSdfp for [[MVS/XA]] and [[MVS/ESA]] systems. A PDS/E library can store program objects or other types of members, but not both. BPAM cannot process a PDS/E containing program objects.