Storage groups, along with storage classes, help reduce the requirement for users to understand the physical characteristics of the storage devices which contain their data.

You can direct new data sets to as many as 15 storage groups, although only one storage group is selected for the allocation. The system uses the storage class attributes, volume and storage group SMS status, MVS™ volume status, and available free space to determine the volume selected for the allocation. In a tape environment, you can also use tape storage groups to direct a new tape data set to an automated or manual tape library.

DFSMShsm uses some of the storage group attributes to determine if the volumes in the storage group are eligible for automatic space or availability management.

Figure 1 is an example of using storage groups to group storage volumes for specific purposes.

Figure 1. Using Storage Groups. In this example, DASD volumes are grouped so that primary data sets, large data sets, DB2 data, IMS data, and CICS data are all separated.

The virtual input/output (VIO) storage group uses system paging volumes for small temporary data sets. The tape storage groups contain tape volumes that are held in tape libraries. The object storage group can span optical, DASD and tape volumes. An object backup storage group can contain either optical or tape volumes within one OAM invocation. Some volumes are not system-managed, and DFSMShsm owns other volumes for use in data backup and migration. DFSMShsm migration level 2 tape cartridges can be system-managed if you assign them to a tape storage group.

You can use data-set-size-based storage groups to help you deal with free-space fragmentation, and reduce or eliminate the need to perform DFSMSdss DEFRAG processing. See Pooling Volumes with Storage Groups for more information.

For objects, there are two types of storage groups: object and object backup. OAM assigns an object storage group when the object is stored. The first time an object is stored to a collection, the storage group ACS routine can override this assignment. You can specify one or two object backup storage groups for each object storage group.

Recommendation: Discourage users from directly requesting specific devices. Unlike data, storage, and management classes, users cannot specify a storage group when allocating a data set, although they can specify a unit and volume. Whether or not you honor their unit and volume request is your decision, but it is more effective for your users to specify the logical storage requirements of their data by storage and management class, which you can then verify in the automatic class selection routines.