Class transition processing moves data sets from one tier of L0 volumes to another. For this strategy to work, enough available space must exist on the L0 volumes that are the targets of the class transition processing. For this reason, DFSMS has provided a new storage group processing priority setting. The value of this priority is from 1 to 100, with a default value of 50. For space management processing, DFSMShsm will process storage groups in the order of their processing priority. Storage groups that will have data sets moved to them as a part of class transition processing should have a relatively higher processing priority such that free space is created for the transitions.