Using Automatic Class Selection Routines

ACS routines determine if a data set is system managed and which classes are to be used.

You can use a storage class and a management class only with system-managed data sets. You can use a data class for data sets that are either system managed or not system managed, and for data sets on either DASD or tape volumes. SMS can manage tape data sets on physical volumes in a tape library and on the logical volumes in a Virtual Tape Server (VTS). DFSMSrmm provides some services for the stacked volumes contained in a Virtual Tape Server (see z/OS DFSMSrmm Implementation and Customization Guide). Your storage administrator defines the data classes, storage classes, and management classes your installation will use. Your storage administrator provides a description of each named class, including when to use the class.

Recommendation: Your storage administrator must code storage class ACS routines to ensure data sets allocated by remote applications using distributed file management are system managed.

Using a data class, you can easily allocate data sets without specifying all of the data set attributes normally required. Your storage administrator can define standard data set attributes and use them to create data classes, for use when you allocate your data set. For example, your storage administrator might define a data class for data sets whose names end in LIST and OUTLIST because they have similar allocation attributes. The ACS routines can then be used to assign this data class, if the data set names end in LIST or OUTLIST.

You can request a data class explicitly, by specifying it in the DD statement, DYNALLOC macro, the TSO or IDCAMS ALLOCATE command, or the DEFINE CLUSTER command. Request a data class implicitly, by not specifying a data class and letting the ACS routines assign the data class defined by your storage administrator. Whichever method is used, you need to know:

The data set characteristics
The data class that describes what this data set looks like
Whether the ACS routines will pick this data class automatically
Which characteristics to code in the JCL to override the data class attributes

You can override any of the attributes specified in the assigned data class by specifying the values in the DD statement, or the ALLOCATE or the DEFINE CLUSTER commands. See z/OS DFSMS Access Method Services Commands (ALLOCATE and DEFINE CLUSTER command sections) for information about the attributes that can be assigned through the SMS class parameters, and examples of defining data sets.

Another way to allocate a data set without specifying all of the data set attributes normally required is to model the data set after an existing data set. You can do this by referring to the existing data set in the DD statement for the new data set, using the JCL keywords LIKE or REFDD. See z/OS MVS JCL Reference and z/OS MVS JCL User's Guide.