What is the storage management subsystem?

DFSMShsm is a member of the DFSMS product family. DFSMS, along with resource access control facility (RACF®, a component of the Security Server for z/OS) and interactive storage management facility (ISMF) licensed programs, provides a system-managed storage environment freeing the user of many time-consuming storage tasks.

The Storage Management Subsystem (SMS) is part of DFSMS. SMS changes the storage management approach from user-managed volumes to SMS-managed data sets residing in SMS-managed storage groups. The system, rather than the user, determines data placement and handles data backup, movement, space, and security.

SMS provides the following storage classes and groups, which are customized by the storage administrator to fit the system environments and policies:

Data class: A list of allocation attributes that the system uses for the creation of data sets.
Storage class: A list of storage performance and availability service requests.
Management class: A list of data set migration, backup, and retention attributes that DFSMShsm uses to manage storage at the data set level.
Storage group: A list of real DASD volumes, or a list of serial numbers of volumes that no longer reside on a system but that end users continue to refer to in their JCL.

Storage administrators also customize SMS automatic class selection (ACS) routines, which automatically assign the SMS classes and storage groups to data sets. This allows the user to create data sets by providing only a small amount of information through ISMF panels.

Because the SMS management class defines the data set migration, backup, and retention parameters for each data set that is SMS-managed, DFSMShsm works with SMS, using the SMS attributes, to provide space and availability management.

There are some differences in the way in which DFSMShsm works in an SMS environment as opposed to a non-SMS environment. Some of those differences are visible to the user when entering explicit commands, as certain commands and parameters do not apply to SMS-managed data sets. Those differences are defined for the user in the "DFSMShsm User Tasks" section of this publication.