When a non-VSAM dataset is created, the user has the option, by means of the DISP=(,CATLG) JCL entry, of creating a catalog entry for the dataset. The catalog keeps track of the unit and volume on which the dataset resides and can be used for later retrieval of the dataset. With VSAM datasets, creation of a catalog entry to record the unit and volume, as well as many other characteristics of the dataset, is not optional.
Prior to VSAM, catalog entries for non-VSAM datasets were contained in OS CVOLS (operating system control volumes). VSAM maintains its own catalog, which is itself a KSDS cluster, into which catalog entries describing VSAM clusters are recorded. The same VSAM catalog may also be used to contain the catalog entries for non-VSAM datasets.
Later releases of OS/390, the operating system into which MVS evolved, and z/OS, the current incarnation of MVS-OS/390, use yet another catalog system - the Integrated Catalog Facility. On the latest versions of OS/390 and z/OS, ICF catalogs are the only type of catalogs supported.
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