With z/OS V2R1, a MIGRATIONSUBTASKS(YES | NO) parameter has been added to the SETSYS command. It allows DFSMShsm to run multiple subtasks concurrently under each migration task for primary space management, on-demand migration, and interval migration on level 0 volumes that migrate data sets to ML1 or ML2 volumes.
The ADDITIONALSUBTASKS(nn) subparameter allows you to dynamically change the number of additional subtasks that the system can use, running under each migration task. These additional subtasks add to the number of subtasks that the system already uses when the MIGRATIONSUBTASKS parameter is specified. Note that the actual number of total subtasks used can vary. In general, the total migration subtasks used will be lower if a large value is specified with the maximum migration tasks (MAXMIGRATIONTASKS) parameter. Conversely, the total migration subtasks used will be higher, up to 15, if a smaller value is specified with the maximum migration tasks (MAXMIGRATIONTASKS) parameter.
For more information, refer to the topic covering the SETSYS command in z/OS DFSMShsm Storage Administration.