CATALOGS - IBM Mainframe

MVS provides a comprehensive catalog facility that records the location of files so that you do not have to specify the vol-ser of the volume that contains the file. Considering that many MVS installations have hundreds of DASD volumes and thousands of data sets, cataloging is a must. It would be impractical to keep track of the locations of that many files manually.

Under MVS, there are two types of catalogs as shown in the following figure: master catalogs and user catalogs. Each MVS system has just one master catalog and an unlimited number of user catalogs. The master catalog contains entries that identify system data sets; that is, data sets that are required for the operating system to function. In contrast, the user catalogs contain entries that identify user data sets that contain user data. In addition, each user catalog must be defined in the master catalog.

An MVS installation can create catalogs using any of three formats. Catalogs of the oldest format, called OS catalogs or CVOLs, are not used much anymore. (CVOLs did not distinguish between master and user catalogs.) When VSAM was announced in the early 1970s, one of its major benefits was a comprehensive catalog facility that could easily be used to manage the growing number of data sets typical of a large MVS installation. This new facility used VSAM catalogs that were based on the master/user catalog structure.

Unfortunately, the VSAM catalog structure contained fundamental weaknesses that led IBM to develop yet another catalog structure, called the Integrated Catalog Facility, or ICF. ICF catalogs are functionally similar to VSAM catalogs, but they have a different internal format. Today, most of the larger installations have fully converted to ICF catalogs, though some smaller installations still use VSAM and CVOL catalogs.

All files managed by VSAM must be cataloged in a VSAM or ICF catalog. For VSAM files, the catalog contains not just information that locates the file, but information that specifies the file's characteristics as well. Since that information for non-VSAM files is stored in the data set labels in the VTOC, non-VSAM files do not have to be cataloged. Still, cataloging a non-VSAM file in a VSAM or ICF catalog makes the file easier to locate later on because you do not have to remember the vol-ser of the volume that contains the file. If your installation uses the Storage Management Subsystem under MVS/ESA, all data sets have to be cataloged.

Master Catalog, User Catalog and Data Sets

As mentioned earlier, the high-level qualifier of a data set name can have special significance. That is because it normally indicates the catalog in which the file is defined. For example, a data set named ALEXIS.COBPROGS.FILES is cataloged in the user catalog indicated by ALEXIS. In some cases, the high-level qualifier and the user catalog name are the same. More often, though, the high-level qualifier is an alias of the actual name. For example, ALEXIS might be an alias for a catalog named VCAT.MPS800. Then, ALEXIS.COBPROGS.FILES would be cataloged in the catalog named VCAT.MPS800. By using aliases, files with different high-level qualifiers can be cataloged in the same user catalog.