Data set (IBM mainframe): Difference between revisions

Revision as of 10:09, 5 July 2009

Revision as of 21:31, 19 October 2009

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Besides members, a PDS consists also of their directory. Each member can be accessed directly using the directory structure. Once a member is located, the data stored in that member is handled in the same manner as a PS (sequential) data set.

Whenever a member is deleted, the space it occupied is unusable for storing other data. Likewise, if a member is re-written, it is stored in a new spot at the back of the PDS and leaves wasted “dead” space in the middle. The only way to recover “dead” space is to perform frequent file compression, that 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 disk in order to use the directory structure to access individual members, not on tape. They are most often used for storing multiple JCL files, 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 [[MVS/XA]] system.