The primary means of managing space in z/OS® is through the Data Facility Storage Management Subsystem (DFSMS™), which comprises a suite of related data and storage management products. DFSMS performs the essential data, storage, program, and device management functions of the system.

In a z/OS system, space management involves the allocation, placement, monitoring, migration, backup, recall, recovery, and deletion of data sets. These activities can be done either manually or through the use of automated processes. When data management is automated, the operating system determines object placement and automatically manages data set backup, movement, space, and security. A typical z/OS production system includes both manual and automated processes for managing data sets.

Depending on how a z/OS system and its storage devices are configured, a user or program can directly control many aspects of data set usage, and in the early days of the operating system, users were required to do so. Increasingly, however, z/OS customers rely on installation-specified settings for data and resource management, and space management products, such as DFSMS, to automate the use of storage for data sets.

Data management includes these main tasks:

• Setting aside (allocating) space on DASD volumes.
• Automatically retrieving cataloged data sets by name.
• Mounting magnetic tape volumes in the drive.
• Establishing a logical connection between the application program and the medium.
• Controlling access to data.
• Transferring data between the application program and the medium.

DFSMS, together with hardware products and installation-specific settings for data and resource management, provides system-managed storage in a z/OS environment.

The heart of DFSMS is the Storage Management Subsystem (SMS). Using SMS, the system programmer or storage administrator defines policies that automate the management of storage and hardware devices. These policies describe data allocation characteristics, performance and availability goals, backup and retention requirements, and storage requirements for the system. SMS governs these policies for the system, and the Interactive Storage Management Facility (ISMF) provides the user interface for defining and maintaining the policies.

The data sets allocated through SMS are called system-managed data sets or SMS-managed data sets. When you allocate or define a data set to use SMS, you specify the data set requirements through a data class, a storage class, and a management class. Typically, you do not need to specify these classes because a storage administrator has set up automatic class selection (ACS) routines to determine which classes are used for a given data set.

DFSMS provides a set of constructs, user interfaces, and routines (using the DFSMS products) to help the storage administrator. The core logic of DFSMS, such as the ACS routines, ISMF code, and constructs, resides in DFSMSdfp™. DFSMShsm™ and DFSMSdss™ are involved in the management class construct. With DFSMS, the z/OS system programmer or storage administrator can define performance goals and data availability requirements, create model data definitions for typical data sets, and automate data backup. DFSMS can automatically assign, based on installation policy, those services and data definition attributes to data sets when they are created. IBM® storage management-related products determine data placement, manage data backup, control space usage, and provide data security.