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This is part three of the series that began with
What is System-Managed Storage?
One of the mechanisms DFSMS/MVS provides to help automate storage management is the SMS constructs. They are defined using ISMF and are entities which are associated with data sets and objects based on an installation-defined policy. The intent behind these constructs is to separate the
logical view of data from the
physical view.
The logical view of data is concerned with what the data looks like and what services it requires. This view is supported by data class, storage class, and management class. The physical view is concerned with where the data actually resides. It is supported by storage group. There is also a construct called the base configuration which is not associated with individual data sets or objects. It contains both logical and physical information and applies to the system as a whole.
These constructs together with additional constructs and information comprise the SMS configuration. The additional constructs and information will not be discussed in this article. For more information, see the
DFSMSdfp Storage Administration Reference on the
DFSMS/MVS Publications page.
Data Class
Data classes apply to both SMS-managed and non-SMS managed data sets. They do not apply to objects. They allow you to define allocation defaults and simplify allocations by using data class in place of many keywords. Data classes can supply such information as space parameters, DCB attributes, VSAM attributes, volume count, and tape attributes.
Any values you explicitly specify always override values specified in a data class. This is to prevent the system from modifying the intent of your allocation. In addition, since not all attributes apply to every data set organization, SMS uses only those data class attributes that have meaning for the data set being allocated.
Storage Class
Storage classes apply only to SMS-managed data sets and objects. They allow you to define different levels of performance and availability services for your data sets. Using them, you can separate the level of service needed by a data set or object from its physical characteristics. Storage classes can supply such information as attributes for dynamic cache management, sequential data set striping, and concurrent copy.
It is the association of a storage class with a data set or object which causes the data set or object to be SMS-managed. Because of this, such functions as dynamic cache management and sequential data set striping apply only to SMS managed data sets. Data sets may be SMS-managed or non-SMS managed. Objects must be SMS-managed.
Management Class
Management classes apply only to SMS-managed DASD data sets and objects. They supply a list of data set migration, backup, and retention values for DASD data sets, as well as backup requirements and class transition criteria for objects. DFSMShsm uses the attributes defined in the management class for a data set to automatically provide both storage management and availability management. Management classes can supply such information as expiration attributes, migration attributes, and backup attributes.
Management classes are what allows system managed storage to provide space management and availability management at the data set level. An SMS-managed DASD data set may or may not have a management class. An object must have a management class.
Storage Group
Storage groups apply only to SMS-managed data sets and objects. They represent the physical storage on which the data sets and objects reside. The physical groupings of devices, or pools, can be modified dynamically without the need for an IPL. There are six types of storage groups:
- Pool:
Contains SMS-managed DASD volumes and allows those volumes to be managed as a single entity
- Dummy:
Contains volume serials of volumes no longer connected to the system which are treated as SMS-managed; allows existing JCL to function unchanged
- VIO:
Contains no volumes; allocates data sets to paging storage which simulates the activity of a DASD volume
- Object:
Contains optical volumes and DASD volumes used for objects
- Object Backup:
Contains optical volumes used for backup copies of objects
- Tape:
Contains SMS-managed private tape volumes
All SMS-managed data sets and objects must have a storage group. Pool storage groups are of particular interest because they allow DASD volumes to be pooled and managed collectively. You can also define whether or not automatic migration, backup, and dump are allowed within the pool.
Base Configuration
There is
one base configuration per SMS configuration. It identifies the systems that the SMS configuration is to manage. It also contains the following installation defaults:
- Default management class:
the name of the management class used to manage those SMS-managed data sets which do not have a management class assigned
- Default unit:
the generic or esoteric unit name which is to be used for non-SMS managed allocations when no unit is specified
- Default device geometry:
the default number of bytes per track and tracks per cylinder used in converting track and cylinder requests to KB and MB when no unit is specified
In Summary
DFSMS/MVS provides the key to system-managed storage in an MVS environment. By using the SMS constructs, you can separate the logical requirements of data from the physical requirements, thereby making you and your users less dependent on the specific hardware configuration. This can simplify the removal and installation of hardware devices. In addition, the SMS constructs allow you to specify service levels for data sets at the data set level and let the system automatically manage them for you.
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