A catalog consists of two separate kinds of data sets: a basic catalog structure (BCS); and a VSAM volume data set (VVDS). The BCS can be considered the catalog, whereas the VVDS can be considered an extension of the volume table of contents (VTOC).

The basic catalog structure is a VSAM key-sequenced data set. It uses the data set name of entries to store and retrieve data set information. For VSAM data sets, the BCS contains volume, security, ownership, and association information. For non-VSAM data sets, the BCS contains volume, ownership, and association information.

The VSAM volume data set is a VSAM entry-sequenced data set. A VVDS resides on every volume that contains a catalog or an SMS-managed data set that is cataloged. It contains the data set characteristics, extent information, and the volume-related information of the VSAM data sets cataloged in the BCS. If you are using the Storage Management Subsystem (SMS), the VVDS also contains data set characteristics and volume-related information for the non-VSAM, SMS-managed data sets on the volume.

The Volume Table of Contents and the VTOC index are system data sets that maintain extent and allocation information for a volume. The VTOC is used to find empty space for new allocations and to locate non-VSAM data sets. For all VSAM data sets, and for SMS-managed non-VSAM data sets, the VTOC is used to obtain information not kept in the VVDS.

VVDS records for VSAM data sets are called "VSAM volume records" (VVRs). Those for SMS-managed non-VSAM data sets are called "non-VSAM volume records" (NVRs). If a non-VSAM data set spans volumes, its NVR is in the VVDS of the data set's first volume. Because a BCS is a VSAM data set, it also has a VVR in the VVDS.

Every catalog consists of one BCS and one or more VVDSs. A BCS does not "own" a VVDS: more than one BCS can have entries for a single VVDS. Every VVDS that is connected to a BCS has an entry in the BCS.

For example, Figure 1 shows a possible relationship between two BCSs and three VVDSs on three disk volumes. "BCS.A" has entries for data sets residing on each of the three volumes. "BCS.C" has entries for data sets residing on volumes B and C. Because each volume has data sets cataloged, each volume contains a VVDS.

BCS.A resides on volume A with VVDS.A. Both the VVDS and the BCS have entries for each other. All three VVDSs are cataloged in BCS.A. BCS.C, residing on volume C, contains entries for VVDS.C and VVDS.B.

Notice that a VVDS has entries for all VSAM and SMS-managed data sets on its volume, whereas a BCS can have entries for data sets residing on any volume.

Figure 1. Relationship of the BCS and the VVDS. Data set names in parentheses are entries in the BCS or VVDS. Only selected arrows are drawn. See the text for a complete explanation of the relationships diagrammed in this figure.

Figure 1 illustrates how data set entries are contained in both the VVDS and the BCS. Information about a data set is also contained in the VTOC of the volume on which the data set resides, even if the data set is cataloged. To successfully perform all possible operations on a cataloged data set using the catalog, all three elements, the VVDS, BCS, and VTOC, must be synchronized. That is, any equivalent information contained in the BCS and VVDS entries for the data set, and the VTOC DSCB for the data set, must be the same. This is normally done automatically. However, if the catalog components, and the VTOC, become unsynchronized, see Analyzing Catalogs for Errors and Synchronization for information on identifying and correcting the problems.

A configuration of catalogs depends on a master catalog. A master catalog has the same structure as any other catalog. What makes it a master catalog is that all BCSs are cataloged in it, as well as certain data sets called "system data sets" (for instance, SYS1.LINKLIB and other "SYS1" data sets). Master catalogs are discussed in The Master Catalog.