If you code DISP=(NEW,CATLG) for a system-managed GDG, when the system allocates the data set, the system catalogs a new generation in a deferred roll-in state. When the system performs end-of-job step processing, the system rolls the deferred generation data set into the GDG. Generation data sets can be in a deferred roll-in state if the job never reached the end-of-job step or if they are allocated with a DISP=(NEW,KEEP). Generation data sets in a deferred roll-in state can be referred to by their absolute generation numbers. You can use the access method services command ALTER ROLLIN to roll in these generation data sets.

The attributes specified for the GDG determines what happens to the older generations when a new generation is rolled. The access method services command DEFINE GENERATIONDATAGROUP creates a GDG. It also specifies the limit (the maximum number of active generation data sets) for a GDG, and specifies whether all or only the oldest generation data sets should be rolled off when the limit is reached.

When a GDG contains its maximum number of active generation data sets, and a new generation data set is rolled in at the end-of-job step, the oldest generation data set is rolled off and is no longer active. If a GDG is defined using DEFINE GENERATIONDATAGROUP EMPTY, and is at its limit, then, when a new generation data set is rolled in, all the currently active generation data sets are rolled off.

The parameters you specify on the DEFINE GENERATIONDATAGROUP command determines what happens to rolled off generation data sets. For example, if you specify the SCRATCH parameter, the generation data set is scratched when it is rolled off. If you specify the NOSCRATCH parameter, the rolled off generation data set is recataloged as rolled off and is disassociated with its GDG.

The access method services command ALTER LIMIT can increase or reduce the limit for an existing GDG. If a limit is reduced, the oldest active generation data sets are automatically rolled off as needed to meet the decreased limit. If a change in the limit causes generations to be rolled off, then the rolled off data sets are listed with their disposition (uncataloged, recataloged, or deleted). If a limit is increased, and there are generation data sets in a deferred roll-in state, these generation data sets are not rolled into the GDG. The access method services command ALTER ROLLIN can be used to roll the generation data sets into the GDG in active status.

For more information about using the access method services commands DEFINE GENERATIONDATAGROUP and ALTER see z/OS DFSMS Access Method Services Commands.