Specifying the maximum automatic volume space management tasks and subtasks

DFSMShsm allows up to 15 automatic volume space management (primary space management, interval migration, and on-demand migration) tasks to run concurrently in each DFSMShsm host during space management. Use the MAXMIGRATIONTASKS parameter of the SETSYS command to control the maximum number of tasks that you want to run concurrently. If you do not specify this parameter on any SETSYS command, the default value for the maximum number of concurrent automatic volume space management tasks is 2.

Example: You can add the following command to the ARCCMDxx parmlib member of SYS1.PARMLIB of each DFSMShsm hosts to handle automatic volume space management:

SETSYS MAXMIGRATIONTASKS(5)

If migration is to tape, each concurrent primary space management task that migrates a data set directly to tape requires a tape unit. If you are using the duplex tape option, two tape units per task are required. Tape units are allocated when a task encounters the first data set that is being migrated to tape, and the task does not release the tape units until it has completed processing all volumes. If migration is to SDSP data sets, each concurrent primary space management task being performed needs an SDSP data set. An SDSP data set is allocated when a task encounters the first data set that is being migrated to an SDSP. The task does not release the SDSP until the entire volume has been processed unless it remains unused for 30 seconds or the functions RECALL or ABACKUP need the SDSP data set.

Note: The actual number of migration tasks can be equal to the number of tasks specified on the MAXMIGRATIONTASKS or the MAXINTERVALTASKS and up to twice the specified number of tasks because of DFSMShsm's preprocessing enhancement. This is reflected in a number of ARC0161I messages issued in response to the QUERY ACTIVE or QUERY ACTIVE(TCB) commands. MIGRATING, MIGRATION PREPROCESSING, FREEVOL PROCESSING, and DBA/DBU PROCESSING differentiate the type of space management DFSMShsm is performing and these are displayed in the ARC0161I message.

Migration tasks perform all space management tasks, including class transitions. Class transitions have a higher priority than migrations and will be processed first during the data movement phase. If a data set is eligible for both a class transition and migration, then it will be migrated.

DFSMShsm can also 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. This can improve throughput by reducing the elapsed time for any migration function involving multiple data sets. Use the MIGRATIONSUBTASKS parameter of the SETSYS command to enable this function.

If you wish to specify additional subtasks under each migration task, you can do this dynamically with the ADDITIONALSUBTASKS(nn) subparameter. Up to 10 additional subtasks are allowed. Specifying a value of zero will remove all previously-defined additional subtasks.

Example: You can add the following command to the ARCCMDxx parmlib member of SYS1.PARMLIB of each DFSMShsm host to enable migration subtasks, and also specify two additional subtasks for each migration task:

SETSYS MIGRATIONSUBTASKS(YES ADDITIONALSUBTASKS(2))

Note that specifying an ADDITIONALSUBTASKS(nn) value greater than 0 should be done with caution, as the additional concurrent subtasks may increase the amount of below the line storage used by DFSMShsm.

Note, also, that when MIGRATIONSUBTASKS(YES) is specified, concurrent cell pool activity may increase in cell pool 5. It is therefore recommended to review the value specified by the CELLS parameter in the DFHSM startup procedure for CELL POOL 5. See "Specifying the size of cell pools" and "Adjusting the size of cell pools" in the z/OS DFSMShsm Implementation and Customization Guide for more information.