DASD
Direct access storage device, the mainframe term for disk storage.
Also written disk, direct access storage device
DASD, pronounced "dazz-dee", stands for direct access storage device. It means disk. The term is old, coined to distinguish disk from tape, where reaching a piece of data means winding through everything before it.
Modern mainframe DASD is not a spinning disk in the way the name suggests. It is a large storage array, often solid state, that presents itself to z/OS as a set of volumes with traditional characteristics so that decades of software continues to work unchanged. The abstraction is deliberate.
You will meet the word constantly in conversation and documentation: datasets live on DASD, a job fails because DASD is full, a site adds DASD capacity. Treat it as a synonym for disk storage and you will not go wrong.
Space on it is organised into volumes, each with a six-character label, and datasets are allocated space in units inherited from disk geometry: tracks and cylinders. Those units are another piece of vocabulary that survives long after the hardware it described.
Related terms
- VolumeA single unit of disk storage with a six-character label, on which datasets are placed.
- DatasetWhat a mainframe calls a file: a named collection of records with a defined structure, rather than a loose stream of bytes.
- Space allocationReserving disk space for a dataset up front, in tracks, cylinders or blocks, with room to grow.
- CatalogThe system's index of where datasets live, so you can refer to a dataset by name without knowing which disk it is on.