What is space management?

Space management is a process that ensures that there is enough storage space on user volumes for your data.

Space management performs two functions that are visible to you:

It expires data sets that have passed their expiration dates. For non-SMS-managed data sets, you specify the expiration date when you allocate the data set. For SMS-managed data sets, the expiration date is controlled by the management class associated with the data set. To find out if your data sets are SMS-managed and what management classes are associated with your data sets, ask your computing center.
It migrates data sets that have not been used within a specified time. A migrated data set is one that has been moved to a different volume (either DASD or tape) in a special form that saves space on the volume. You cannot use a migrated data set directly. However, as soon as you refer to the data set, either in an interactive or batch use, DFSMShsm returns the data set to a volume that you are authorized to use and in the form in which it originally existed.

You can recognize that a data set has been migrated by the term MIGRAT in the volume field when you list your cataloged data sets. Only cataloged data sets can be migrated.

The amount of time that a data set remains unused before it can migrate automatically is the same for all data sets on a volume when they are non-SMS-managed. For SMS-managed data sets, the management class determines when the data sets are eligible to automatically migrate. Your computing center can tell you what these values are.

Migration occurs to either of two levels: migration level 1 or migration level 2. Migration level 1 (ML1) volumes are always DASD. Migration level 2 (ML2) volumes can be either DASD or tape. Your computing center controls which volumes are to be used as migration volumes.

DFSMShsm creates available space on user volumes by:

Freeing over allocated space
Deleting expired data sets
Moving eligible data sets that you have not used recently to a lower-cost-per-byte storage device

DFSMShsm records the location of each data set that it moves in a control data set.