What Are Automatic Class Selection (ACS) Routines?

DFSMS: What is System Managed Storage? Part 6

This is part six of the series that began with What Is System-Managed Storage?.

What Is an ACS Routine?

The ACS routines are one of the mechanisms DFSMS/MVS provides to help automate storage management. Using ACS routines, you can centralize control of storage management and automatically associate SMS classes and groups with data sets. For more information, see the DFSMSdfp Storage Administration Reference.

An ACS routine is a sequence of instructions for having the system assign SMS classes and groups to data sets and objects. The selection of specific classes and groups is based on information from JCL or other allocation parameters. ACS routines can use parameters, such as data set name, volume serial number, job name, and data set size, to assign classes and groups to data sets.

There are four types of ACS routines. There can be one ACS routine of each type in an SMS configuration. ACS routines are executed whenever data sets and objects are created, either at initial allocation or due to an operation such as RECALL, RECOVER, COPY, or RESTORE. They are executed in the following order:

1. data class

   The data class routine can assign data classes for both SMS-managed and non-SMS-managed data sets. This allows users to simplify allocations by using supplied values for JCL parameters or DCB attributes. The data class ACS routine is run only at initial allocation.

2. storage class

   The storage class routine can assign storage classes for both data sets and objects. Objects must be SMS-managed; data sets may be SMS-managed or non-SMS-managed. Any DASD data set which is assigned a storage class is considered to be SMS-managed.

3. management class

   The management class routine is run only if a storage class is assigned to the data set or object. Management classes apply only to SMS-managed data sets and objects.

4. storage group

   The storage group routine is run only if a storage class is assigned to a data set. Of all the ACS routines, only the storage group ACS routine is required. The storage group assigned to a data set or object determines where the data set or object resides. All SMS-managed data sets and objects must be assigned a storage group.

Why Should I Write ACS Routines?

Automatic class selection provides centralized control over data set allocation on SMS-managed volumes. If SMS is activated, all new data set allocations are subject to automatic class selection. While data class, storage class, and management class can be explicitly specified, it is the ACS routines which are the final arbiters. They may allow the use of the explicitly specified class, assign a different class, return a null value, or even fail the allocation by exiting with a non-zero code.

You must provide a storage group ACS routine because storage groups cannot be explicitly specified. The ACS routine is the only mechanism for associating storage groups with data sets and objects. This is to help remove the need for users to be aware of the physical storage environment. This in turn allows you to modify the physical storage environment without having to involve the user community.

The storage group ACS routine must return the name of a valid storage group. If it does not, the allocation will fail since the system will be unable to determine where the data set or object should be placed.

Converting Data to SMS-managed Storage

ACS routines can determine whether a new data set should be SMS-managed. You can define your ACS routines to direct selected data sets to SMS-managed DASD or tape volumes; while objects can be directed to optical volumes. Using the ACS routines, you can convert as much or as little data as you want at a time.

As the conversion to SMS-managed storage proceeds, you can modify the ACS routines to select more data sets to be SMS-managed. In this way, conversion to SMS-managed storage can take place gradually, minimizing user involvement and modifications to JCL or other allocation statements. You can also use DFSMSdss to convert existing data sets to SMS-managed storage without data movement.

In Summary

DFSMS/MVS provides the key to SMS-managed storage in an MVS environment. Using the ACS routines you can automatically make as much or as little data SMS-managed as is appropriate for your installation. This allows you to implement system-managed storage with minimal JCL changes and little impact to end users. It also allows you to experiment with system-managed storage and use specific data sets to demonstrate the benefits of making data sets SMS-managed.

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