DASD

DASD stands for direct access storage device, a class of data storage devices that allow read and write operations to fixed storage blocks located on a disk or similar non-volatile medium. Unlike sequential storage such as magnetic tape, a DASD provides random access to data, enabling retrieval and updates of arbitrary blocks without reading through preceding data.

In computing environments that include mainframes and enterprise servers, DASD typically refers to disk storage subsystems connected to a computer through dedicated I/O channels and control units. IBM’s mainframe systems popularized the term; DASD has historically encompassed disk packs and later disk arrays integrated into storage subsystems. Two addressing schemes have been used on IBM DASD: Extended Count Key Data (ECKD) and Fixed Block Architecture (FBA). ECKD uses records and keys for advanced data management, while FBA presents blocks in a simple fixed layout.

Modern DASD often appears as logical volumes presented by storage controllers or virtualization layers, with features such as caching, error checking, and redundancy. In practice, DASD can be organized into volumes and managed via storage management software, with RAID or mirroring providing fault tolerance. As computing evolved, DASD has become part of larger storage area networks and cloud-backed infrastructures, but its core characteristic remains direct, block-level access to data.

Outside of mainframe contexts, the term DASD is sometimes used more generally to describe any disk attached directly to a host, as opposed to network-attached or removable media.