Direct Access Storage Device
In mainframe computers and some minicomputers, a direct access storage device, or DASD ( /ˈdæzdiː/), is any secondary storage device which has relatively low access time relative to its capacity.
Historically, IBM introduced the term to cover three different device types:
- disk drives
- magnetic drums
- data cells
The direct access capability, occasionally and incorrectly called random access (although that term survives when referring to memory or RAM), of those devices stood in contrast to sequential access used in tape drives. The latter required a proportionally long time to access a distant point in a medium. Note: The storage CLASS of DASD is both Fixed and Removable. The access methods for DASD are Sequential, Indexed and Direct.
... Both drums and data cells have disappeared as products, so DASD remains as a synonym of a disk device ... Modern DASD used in mainframes only very rarely consist of single disk-drives most commonly "DASD" means large disk arrays utilizing RAID schemes ...
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