Understanding data classes

A data class is a list of data set allocation attributes and their values. You cannot assign a data class to an object; however, data class may be used for allocation of a scratch tape to be used to write objects. When end users allocate a data set and refer to a data class either explicitly (for example, through JCL) or implicitly (through ACS routines), SMS allocates the data set using the attribute values of its associated data class. For data class attributes, explicit specifications by end users override any parameters derived from the data class ACS routine. If SMS is active, a data class can be assigned to any data set. For non-SMS-managed DASD data sets, the system uses the allocation attribute values of the data class, but it does not save the data class name. For tape data sets, only the expiration and retention values are applied. Specifying the Override Space attribute causes the space attributes from the data class to override the space information specified explicitly in the JCL.

Not all attributes apply to every data set organization. When SMS allocates a data set, it uses only those data class attributes that have meaning for the given data set organization. SMS saves the data class name for each SMS-managed data set. The actual data class definitions reside in the SCDS. If you alter a data class definition, SMS applies the changes to any new data sets that use the data class after you activate the changed configuration. However, SMS does not retroactively apply your changes to previously allocated data sets. To apply your changes to existing data sets, you need to delete and redefine the data sets.

A data class definition contains identification and allocation information. To identify and refer to your data classes, you assign each one a unique name that contains from one to eight alphanumeric characters. Each data class maintains an owner ID that identifies the storage administrator who originally created or last modified the data class. The owner ID can be viewed on the Data Class List panel. Also, each data class contains an optional 120 character description field for describing its contents.

The data class allocation attributes match the keywords that you use to allocate data sets. The attributes contain space-related, access-related, and organizational information that you typically find on JCL DD statements, TSO/E ALLOCATE commands, access method services DEFINE commands, dynamic allocation requests, and ISPF/PDF panels.