Nearly all work in the system involves data input or data output. In a mainframe, the channel subsystem manages the use of I/O devices, such as disks, tapes, and printers.

The operating system must associate the data for a given task with a device, and manage file allocation, placement, monitoring, migration, backup, recall, recovery, and deletion.

These data management activities can be done either manually or through the use of automated processes. When data management is automated, the system determines object placement, and automatically manages object backup, movement, space, and security. A typical z/OS® production system includes both manual and automated processes for managing data.

Depending on how a z/OS system and its storage devices are configured, a user or program can directly control many aspects of data management, and in the early days of the operating system, users were required to do so. Increasingly, however, z/OS installations rely on installation-specific settings for data and resource management, and add-on storage management products to automate the use of storage. The primary means of managing storage in z/OS is with the Data Facility Storage Management Subsystem (DFSMS™), which comprises a suite of related data and storage management products.