Understanding the Object Access Method

The Object Access Method (OAM) is a component of DFSMSdfp, the base for the z/OS product. OAM uses the concepts of system-managed storage, introduced by z/OS, which provide functions for data and space management. z/OS offers the following advantages to its users:

Facilitates the management of storage growth
Improves the use of storage space
Reduces the effort of device conversion and coexistence
Provides centralized control of external storage
Exploits the capabilities of available hardware

OAM supports a class of data referred to as objects. An object is a named stream of bytes. The content, format, and structure of that byte stream are unknown to OAM. For example, an object can be a compressed scanned image or coded data. Objects are different from data sets handled by existing access methods. The characteristics that distinguish them from traditional data sets include:

Lack of record orientation: There is no concept of individual records within an object.
Broad range of size: An object can contain 1 byte or up to 2000 MB (2 097 152 000 bytes) of data. The maximum object size for the disk and tape levels of the OAM storage hierarchy is 2000 MB. The maximum object size for the optical level of the OAM storage hierarchy is 256 MB (268 435 456 bytes).
Volume: Objects are usually much smaller than data sets; however, they are more numerous and consume vast amounts of external storage.
Varying access-time requirements: Reference patterns for objects change over time or cyclically, allowing less critical objects to be placed on lower-cost, slower devices or media.

z/OS includes the definition of a storage hierarchy for objects and the parameters for managing those objects. OAM uses the z/OS-supplied hierarchy definition and management parameters to place user-accessible objects anywhere in the storage hierarchy.

The location of an object in the hierarchy is unknown to the user. Device-dependent information is not required of the user; for example, there are no JCL DD statements and no considerations for device geometry, such as track size.

OAM provides an application programming interface known as the object storage request (OSREQ) macro to store, retrieve, delete, query, and change information about an object. OAM includes the functions necessary to manage the objects after storing them.

OAM stores objects in collections. A collection is a group of objects that typically have similar performance characteristics:

CHARACTERISTIC: DESCRIPTION
Availability: The degree to which a resource is ready when needed.
Backup: A copy of the information that is kept in case the original is changed, lost or destroyed.
Retention: The default lifetime of an object.
Class transition: An event that can cause the assignment of a new management class, storage class, or both.

A collection is used to catalog a large number of objects, which, if cataloged separately, require an extremely large catalog. Every object must be assigned to a collection. Object names within a collection must be unique; however, the same object name can be used in multiple collections. A collection can belong to only one storage group, even though that storage group can have many collections associated with it.