A GDG contained in a catalog is managed through access method services. The access method services DEFINE command can be used to allocate a GDG and to specify how to handle older and obsolete generations.

A GDG is managed using the information found in a generation index.

Rule: An alias name cannot be assigned to the highest level of a generation index.

The BLDG function of IEHPROGM builds the index. The BLDG function also indicates how older or obsolete generations are to be handled when the index is full. For example, when the index is full, you might want to empty it, scratch existing generations, and begin cataloging a new series of generations. After the index is built, a generation can be cataloged by its GDG name, and by either an absolute generation and version number or a relative generation number.

Examples of how to build a GDG index are found in z/OS DFSMS Access Method Services Commands and in z/OS DFSMSdfp Utilities.