Establishing performance objectives

Different response times are required for different sets of objects and some objects are accessed more frequently than others. OAM uses the storage class to specify object performance objectives and availability requirements to SMS. Every object in the object storage hierarchy must have an associated storage class. The fact that every object has an associated storage class makes every object, by definition, SMS-managed.

Your business needs provide the service-level criteria on which storage classes are built. Table 1 shows how you can specify performance objectives for different storage classes depending on the service levels required:

Table 1. Storage class service levels for a variety of business needs
Business need	Service level
Daily operation	Fast response; frequent access
Online customer inquiries	Fast response; occasional access
Quarterly batch processing	Medium response; periodic access
Legal retention requirements	Slow response; very infrequent access

A storage class does not represent any physical storage. OAM analyzes the storage class parameters and tries to meet the performance objectives by placing the object on a device that best meets those objectives. Using storage classes to force use of a specific device type can defeat the purpose of system-managed storage and cause serious inefficiencies.

Example: Using a storage class that causes objects to be written directly to optical media without being staged through disk can degrade system performance. It also can significantly increase the number of optical disks needed per day because of the inefficient storing of optical volume table of contents (VTOC) information.

Consider separating the storage classes that are used to control objects for one application from the storage classes that are used for other applications. If it becomes necessary to change the performance objectives for objects used by an application, its associated storage classes can be changed without affecting the other applications.

Related reading: For a detailed discussion of storage classes and how to plan for them, see z/OS DFSMSdfp Storage Administration.