Allocation of SMS-managed data sets

If SMS is active, it can handle data set storage and management requirements for you. The storage administrator defines SMS classes with ACS routines, which assign classes to a new data set. When a storage administrator assigns a storage class to a new data set, the data set becomes an SMS-managed data set. Data class and management class are optional for SMS-managed data sets. For information on writing ACS routines, see z/OS DFSMSdfp Storage Administration.

Your storage administrator writes ACS routines that assign SMS classes to a data set. The SMS classes follow:

Storage class Contains performance and availability attributes you can use to select a volume for a data set. You do not need to use the volume and unit parameters for a data set that is SMS-managed.
Data class Contains the attributes related to the allocation of the data set, such as LRECL, RECFM, and SPACE. The data set attributes, if not specified in the ALLOCATE command, are derived from the model specified on LIKE, or from the data class. If the system cannot allocate the requested amount of space on the eligible volumes in the selected storage group, SMS retries allocation with a reduced space quantity. However, SMS will not do any retries, including reduced space quantity, unless Space Constraint Relief = Y is specified. If the data class assigned to the data set allows space constraint relief, other limits can be bypassed.
For a list of the attributes for a data class, see the description of the DATACLAS parameter.
Management class Contains the attributes related to the migration and backup of the data set by DFSMShsm™.