Direct access storage device

In mainframe computers and some minicomputers, a direct access storage device, or DASD (pronounced /ˈdæzdi/), is any secondary storage device which has relatively low access time for all its capacity.

Historically, the term was introduced by IBM to cover three different device types: disk drives, magnetic drums and data cells. The direct access capability, now called random access, of those devices was opposed to sequential access used in tape drives. The latter required very long time to access a distant point in a medium. Both drums and data cells have disappeared as products, so now DASD is a synonym of a disk device. Modern DASD used in mainframes are very rarely single disk drives, most commonly those are large disk arrays utilizing RAID schemes.

Direct access storage device

See also