After putting the candidate data sets in priority order, DFSMShsm compares the occupied space on the volume to the low threshold for the volume. If the occupied space is more than the low threshold, DFSMShsm migrates the highest priority data set. Data set size is a determining factor for the order in which data sets are migrated. Managing an entire extended address volume (EAV) with one threshold is not sufficient because average data set size in the cylinder-managed space will likely exceed the average data set size in track-managed space. This will result in data sets allocated in the cylinder-managed space being migrated at a higher rate than data sets allocated in the track-managed space, potentially causing no space in the track-managed space being freed. To prevent this, a unique threshold for the track-managed space (track-managed threshold) is used to manage the free space separate from the free space on the entire volume (volume-level threshold).
Data sets that fail reconnection remain eligible for normal migration. Data sets that fail reconnection because there is no ability to serialize either the data set or the target ML2 volume are assigned the lowest migration priority.
Data sets are migrated until the occupied space is equal to or less than the low threshold specified by the storage group or until no more migration candidates exist on the volume. At that point, DFSMShsm ends automatic primary space management processing for that volume.
As DFSMShsm selects each data set for migration, it transfers control to the space management exit (ARCMDEXT) if the exit is installed and enabled.
| Eligibility Status | Migration Exit RC=0 (Allow Migration) | Migration Exit RC=8 (Disable Migration) | Migration Exit RC=52 (Disable Migration and Class Transition) |
|---|---|---|---|
| Eligible for migration only | Migration | Data set processing fails with RC=45, reason 92 | Data set processing fails with RC=45, reason 92 |
| Eligible for migration and class transition | Migration | Class transition | Data set processing fails with RC=45, reason 92 |
| Eligible for class transition only | Class transition | Class transition | Data set processing fails with RC=45, reason 92 |
If any data set is directed to tape during DFSMShsm-managed volume space management, a tape volume is allocated and mounted. If you are using the duplex tape option for migration, two tape volumes are allocated and mounted. When a tape volume is allocated for DFSMShsm-managed volume migration, it remains allocated until it becomes full or is needed for some other DFSMShsm function. When the DFSMShsm host finishes DFSMShsm-managed volume migration, DFSMShsm deallocates and demounts the tape volume.