The generation data group (GDG) management attributes let you specify migration eligibility for generation data sets based on relative generation number and what to do with rolled-off generation data sets (GDSs).
In establishing the order for migrating data sets from a DFSMShsm-managed volume, DFSMShsm can give high priority to generation data sets (members of generation data groups). That is, DFSMShsm can choose to migrate generation data sets before it migrates non-generation data sets. Generation data sets that are chosen in preference to non-generation data sets are said to receive early migration. You may want to give high priority to generation data sets if old generations are used infrequently. To specify that old generations have high priority for migration and to specify how many generations have standard priority, you can specify the # GDG ELEMENTS ON PRIMARY attribute. When you specify this attribute, all non-rolled-off generations of each GDG that exceed the number you specify are given priority for early migration. If you specify 0 for this attribute, all generations of the GDGs are given priority for early migration.
The ROLLED-OFF GDS ACTION attribute specifies what to do with rolled-off GDSs. The values for this attribute are MIGRATE and EXPIRE. If you specify MIGRATE, DFSMShsm assigns the highest migration priority to rolled-off GDSs. If you specify EXPIRE, DFSMShsm deletes any rolled-off GDS that has met the expiration date specified in its data set VTOC entry or that has no expiration date specified in the data set VTOC entry. If a NOSCRATCH GDS rolls off while on a level 0 volume and is migrated, the GDS is expired during migration cleanup.
The GDG management attributes have no defaults. If you do not specify values for these attributes, the other migration and expiration attributes in the management class govern migration and expiration of generation data sets. If a GDS does not meet any of the criteria specified by the GDG management attributes, the other migration and expiration attributes in the management class govern migration and expiration of the data set.