Space management function

Space management consists of removing appropriate data sets from the level 0 volumes and reducing the space occupied on the level 0 volumes by the data sets that remain there. In removing the data sets, you can have DFSMShsm either delete them entirely, migrate them to migration DASD or tape volumes, or transition them to other user-accessible DASD volumes. If you move the data sets to migration DASD or tape volumes (ML1 or ML2), they can still be automatically deleted from the migration volumes when they expire.

Rule: If a data set is larger than the free space on the DASD backup or migration volume, then the migration or backup will fail.

Note: DFSMShsm allows use of the cylinder-managed space on extended address volumes (EAVs) for migration copies and backup versions. Use the SETSYS USECYLINDERMANAGEDSPACE command to manage allocation of ML1, ML2, or backup data sets on cylinder managed space. See SETSYS command: Establishing or changing the values of DFSMShsm control parameters for more information.

The space management functions of DFSMShsm for SMS-managed volumes are performed on a data set level basis. That is, for each data set on a volume to be managed, DFSMShsm uses the management class attributes of the management class with which the data set is associated to determine:

Whether to release unused overallocated space
Whether to delete the data set based on:
- The amount of time since the data set was created
- The amount of time since the data set was last used
- An explicit date
The minimum time to leave the data set unopened on the level 0 volume before migrating it
Whether a data set on a level 0 volume is eligible to move to an ML1 volume or directly to an ML2 volume
Whether to move the data sets on ML1 volumes to ML2 volumes
For data sets on ML1 volumes, the minimum time to leave the data set unopened on both the level 0 volume and the ML1 volume before moving it to an ML2 volume
A capability to migrate older generation data groups (GDGs)
Whether to delete or migrate rolled-off generations if the data set was a member of a GDG
Whether to transition the data set from one class of level 0 volumes to another, based on creation date, date since last referenced, or a specific periodic timeframe

DFSMShsm uses the attributes from the storage group to determine which level 0 volumes to process for space management, how full the volume must be before data sets are migrated or transitioned from the volume, and how much free space should be on the volume after space management has finished.

As space management processes each volume, it first tries to make space on the volume by deleting expired and temporary data sets and by releasing over-allocated space in data sets that cannot be deleted. During extent reduction, DFSMShsm releases any unused space in the data set, both internal and on the end, and places the data set on the same volume in as few extents as possible. If deleting data sets and releasing space does not make enough space available, DFSMShsm uses the management class to determine which data sets are eligible to transition to a different level 0 volume and which data sets are eligible to move to a migration volume.

Rule: All ML1 volumes must be DASD. If ML1 volumes become sufficiently full, DFSMShsm moves migrated data sets from ML1 volumes to ML2 volumes as specified by either:

The management class attributes for each data set
The THRESHOLD parameter on the SETSYS ML1OVERFLOW command if you are using ML1 OVERFLOW volumes. See ML1OVERFLOW: Manage ML1 OVERFLOW volumes.

ML2 volumes can be either DASD or tape. You can issue a DFSMShsm command to move data sets from DASD ML2 volumes to tape ML2 volumes. As data sets on tape ML2 volumes become not valid, you can recycle the valid data sets to other tape ML2 volumes.

Recall, the reverse of migration, moves data sets from ML1 or ML2 volumes to level 0 volumes. With the recall function, DFSMShsm returns data sets to storage groups as specified by SMS, whether the data set migrated from a volume in a storage group or not. This allows data sets that migrated before they were SMS-managed to be put under SMS management during recall. If a data set is recalled to an SMS-managed EAV volume, then the EATTR attribute of the data set is taken into consideration for level 0 volume selection.