z/OS DFSMS Using Data Sets
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Processing Generation Data Groups

z/OS DFSMS Using Data Sets

You can catalog successive updates or generations of related data. They are called generation data groups (GDGs). Each data set within a GDG is called a generation data set (GDS) or generation. Within a GDG, the generations can have like or unlike DCB attributes and data set organizations. If the attributes and organizations of all generations in a group are identical, the generations can be retrieved together as a single data set.

There are advantages to grouping related data sets. For example, the catalog management routines can refer to the information in a special index called a generation index in the catalog. Thus:
  • All of the data sets in the group can be referred to by a common name.
  • The operating system is able to keep the generations in chronological order.
  • Outdated or obsolete generations can be automatically deleted by the operating system.

Generation data sets have sequentially ordered absolute and relative names that represent their age. The catalog management routines use the absolute generation name. Older data sets have smaller absolute numbers. The relative name is a signed integer used to refer to the latest (0), the next to the latest (-1), and so forth, generation. For example, a data set name LAB.PAYROLL(0) refers to the most recent data set of the group; LAB.PAYROLL(-1) refers to the second most recent data set; and so forth. The relative number can also be used to catalog a new generation (+1).

A generation data group (GDG) base is allocated in a catalog before the generation data sets are cataloged. Each GDG is represented by a GDG base entry. Use the access method services DEFINE command to allocate the GDG base.

Note: For new non-system-managed data sets, if you do not specify a volume and the data set is not opened, the system does not catalog the data set. New system-managed data sets are always cataloged when allocated, with the volume assigned from a storage group.

See z/OS DFSMS Access Method Services Commands for information about defining and cataloging generation data sets in a catalog.
  1. A GDG base that is to be system managed must be created in a catalog. Generation data sets that are to be system managed must also be cataloged in a catalog.
  2. Both system-managed and non-system-managed generation data sets can be contained in the same GDG. However, if the catalog of a GDG is on a volume that is system managed, the model DSCB cannot be defined.
  3. You can add new non-system-managed generation data sets to the GDG by using cataloged data sets as models without needing a model DSCB on the catalog volume.

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