Data set (IBM mainframe): Difference between revisions

Revision as of 10:40, 19 September 2007

Revision as of 10:48, 5 October 2007

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A '''data set''', or '''dataset''', is a [[computer file]] having a [[record-oriented file|record organization]]. The term pertains to the [[IBM]] [[mainframe computer|mainframe]] operating system line, starting with [[OS/360]], and is still used by its successors, including the current [[z/OS]]. Those systems historically preferred this term over a ''file''. Data set is typically stored on [[direct access storage device]] (DASD) or [[magnetic tape]].

Datasets are not unstructured streams of [[byte]]s, but rather are organized in various logical record and block structures determined by the <code>DSORG</code> (data set organization) and, <code>RECFM</code> (record format), parametersand ofother the [[Data Control Block]] (DCB)parameters. The <code>DSORG</code>, <code>RECFM</code> and similarThese parameters are specified at the time of the data set allocation (creation), for example with the [[Job Control Language]] <code>DD</code> statements. ThenInside a job they are stored in the [[Data Control Block]] (DCB), which is a data structure used to later access datasets, for example using [[access method]]s.

== Dataset Organization ==

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In OS/360, the DCB's DSORG parameter specifies how the dataset is organized. It may be physically sequential ("PS"), indexed sequential ("IS"), partitioned ("PO"), or Direct Access ("DA"). Datasets on tape may only be DSORG=PS. The choice of organization depends on how the data is to be accessed, and in particular, how it is to be updated.

Programmers utilize various ''[[access methods'']] (such as [[QSAM]], [[ISAM]], or [[VSAM]]) in programs reading and writing data sets, their choice depending on given data set organization.

== Record Format (RECFM) ==