ABCs of z/OS System Programming Volume 3 - IBM Redbooks

4.68 DFSMShsm: Automatic space management
HSM.HMIG.ABC.FILE1.T891008.I9012
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Figure 4-79 DFSMShsm automatic space management (migration)
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Automatic space management (migration)
Automatic space management prepares DASD space for the addition of new data by freeing
space on the DFSMShsm-managed volumes (L0) and DFSMShsm-owned volumes (ML1).
The functions associated with automatic space management can be divided into two groups,
namely automatic volume space management and automatic secondary space management,
as explained here.Automatic volume space management
Primary Invoked on a daily basis, it cleans L0 volumes by deleting expired and
temporary data sets, releasing allocated and not used space (scratch).
During automatic primary space management, DFSMShsm can
process a maximum of 15 volume migration tasks concurrently.
This activity consists of the deletion of temporary data sets, deletion of
expired data sets, the release of unused and overallocated space, and
migration. Each task processes its own separate user DASD volume.
The storage administrator selects the maximum number of tasks that
can run simultaneously, and specifies which days and the time of day
the tasks are to be performed.
In z/OS V1R8, there is a specific task to scratch data sets. If after that
the free space is still below a threshold, then it moves data sets (under
control of management class) from L0 to ML1/ ML2 volumes.
Interval migration Executed each hour throughout the day, as needed for all storage
groups. In interval migration, DFSMShsm performs a space check on
Chapter 4. Storage management software 221