Class transition occurs during the second phase of volume-level space management. Data sets that are eligible for a class transition are processed first and are only transitioned if they are not eligible for migration as well. This prevents the data from being transitioned to another volume, only to be later migrated.
When a data set is eligible for a class transition, the SMS ACS routines are invoked with an ACS environment of SPMGCLTR.
The data class ACS routine is not invoked.
The storage class ACS routine is invoked. If DFSMShsm is unable to move the data set to a new storage class, then the data set is not transitioned unless a new storage group has also been assigned. The storage class is derived based on the existing management class and the existing storage group of the data set.
The management class ACS routine is invoked. If the new management class has no transition criteria then no actual data movement is performed. DFSMShsm assigns the data set to the new management class. The management class is derived based on the existing management class and the new storage class assigned.
The storage group ACS routine is invoked. If a new storage group is assigned, DFSMShsm attempts to move the data set to the appropriate storage group. The storage group is derived based on the new management class and the new storage class assigned.