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This is part four of the series that began with
What is System-Managed Storage?
What is space management?
Space management lets you keep DASD space available for users, in order to meet your system's service-level objectives. The purpose is to manage your DASD storage efficiently. To do this, space management automatically and periodically:
- Expires data sets that have passed their expiration dates
- Migrates unused data sets to locations where they are stored less expensively or in a form that saves space on the volume
- Reduces the space occupied by data by freeing unused portions
What can DFSMShsm do for me?
DFSMShsm, a component of DFSMS/MVS, provides:
- Partitioned data set (PDS)compression when data sets are moved or copied
- When migrating, recalling, backing up, or recovering PDSs, DFSMShsm moves only data. It releases both unused space and used space that is no longer valid.
- Deletion of temporary data sets
- DFSMShsm automatically deletes temporary data sets unintentionally left at the end of a task.
- Release of overallocated space
- DFSMShsm automatically frees allocated space that does not contain any data.
- Deletion of expired data sets
- When a data set reaches its expiration date, DFSMShsm determines if the data set is eligible for deletion and, if so, deletes it.
- Extent reduction
- This occurs both during recall or recovery and as a special action taken by DFSMShsm when it detects a data set approaching the 16-extent limit. If the data set is not already migrated or backed up, DFSMShsm migrates it to migration level 1 (ML1) DASD. It then reallocates the data set in as few extents as possible.
- Reblocking
- Reblocking changes the number of records in a physical block to use DASD space more efficiently. DFSMShsm, using DFSMSdss as the data mover, reblocks physical sequential data sets during recall and recovery.
- Expiration of backup versions
- An authorized user can delete backup versions of data sets when their expiration criteria have been reached.
What else can DFSMS/MVS do for me?
Other space management functions available in DFSMS/MVS include the release of overallocated space when a data set is closed and the defragmentation of space on user volumes. The first is provided by DFSMSdfp, the second by DFSMSdss.
Release of overallocated space at close time is supported only for SMS-managed data sets. It applies to non-VSAM data sets and extended format VSAM key-sequenced data sets and is selected at the data set level. The following options are available:
- Release only during space management cycle
- Release during space management cycle and at close if the data set was open for output
- Release during space management cycle only if secondary space was specified
- Release during space management cycle and at close only if secondary space was specified
- Do not release unused space
Defragmentation consolidates free space on volumes. Frequent creation, extension, and deletion of data sets fragment the space on DASD volumes. This can result in:
- Inefficient use of DASD space
- An increase in space-related abends
- Performance degradation due to excessive DASD arm movement
- An increase in the time required for direct access device space management (DADSM) functions
You can consolidate the free space on volumes to avoid these problems. This process relocates data set extents on a DASD volume to reduce or eliminate free space fragmentation. You can also specifically exclude certain data sets.
Why should I use system-managed storage?
Without system-managed storage, you must set some space management parameters individually for each volume you want managed. You can set others only for the system as a whole. Therefore, space management for data sets is based on the volumes on which they happen to reside.
With system-managed storage, you can establish an entire SMS storage group as a candidate for auto-migration. You need not issue special commands to set the necessary parameters; all information is contained in the SMS configuration. In addition, because data sets are managed at the data set level based on their management classes, data sets with different expiration and migration requirements can coexist on the same volumes.
When DFSMShsm performs automatic space management for SMS-managed volumes, it does all of the functions mentioned earlier. For non-SMS managed volumes, it does not:
- Release overallocated, unused space
- Expire data sets by inactive age
The last two functions are handled using management class parameters, and management class applies only to SMS-managed data sets.
Another consideration for releasing overallocated, unused space is whether you want it done when data sets opened for output are closed. This can be useful because:
- It does not require modifying JCL to add the RLSE parameter
- Overallocated space is freed immediately rather than during the next space management cycle
This is another function controlled by management class and therefore applies only to SMS-managed data sets.
In summary
DFSMS/MVS provides the key to system-managed storage in an MVS environment. Using a combination of storage group and management class, you can define a data set level policy for space management. The system then automatically expires and migrates data sets and releases overallocated, unused space. This helps make DASD space usage more efficient and helps reduce the number of out-of-space incidents you experience.
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