What is a Catalog?

A catalog describes data set attributes and indicates the volumes on which a data set is located.

When a data set is cataloged, it can be referred to by name without the user needing to specify where the data set is stored. Data sets can be cataloged, uncataloged, or recataloged. All system-managed DASD data sets are cataloged automatically in a catalog.

Cataloging of data sets on magnetic tape is not required, but doing so can simplify users’ jobs.

In z/OS, the Master Catalog and User Catalogs store the locations of data sets. Both disk and tape data sets can be cataloged. To find a data set that you have requested, z/OS must know three pieces of information:

–    Data set name

–    Volume name

–    Unit (the volume device type, such as a 3390 disk or 3590 tape)

You can specify all three values on ISPF panels or in JCL. However, the unit device type and the volume are often not relevant to an end user or application program. A system catalog is used to store and retrieve UNIT and VOLUME location of a data set. In its most basic form, a catalog can provide the unit device type and volume name for any data set that is cataloged. A system catalog provides a simple look-up function. With this facility the user need only provide a data set name.


Master Catalog and User Catalog

A z/OS system always has at least one Master Catalog. If it has a single catalog, this catalog would be the Master Catalog and the location entries for all data sets would be stored in it. A single catalog, however, would be neither efficient nor flexible, so a typical z/OS system uses a master catalog and numerous User Catalogs connected to it.

A User Catalog stores the name and location of a data set (dsn/volume/unit). The master catalog usually stores only a data set High-Level Qualifier (HLQ) with the name of the user catalog, which contains the location of all data sets prefixed by this HLQ. The HLQ is called an alias.

As a general rule, all user data sets in a z/OS installation are cataloged. Uncataloged data sets are rarely needed and their use is often related to recovery problems or installation of new software. Data sets created through ISPF are automatically cataloged.

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