Storage Area Networks: Designing and Implementing a Mass Storage System

2.3. Disks

Disks are called direct access storage devices (DASD). Although they can store sequential data just like tape, their claim to fame has always been direct access.

Records can be written anywhere on a disk and can be accessed directly from their disk location. There’s no need to read hundreds or thousands of preceding data blocks. That means disk access is fast.

By the way, mainframe manufacturers experimented with drums and data cells as direct access storage methodologies, but they never gained any acceptance to speak of.

Disks drives weren’t cheap, and a small mainframe shop might have four drives, each the size of a washing machine. By contrast, the shop might have eight tape drives. The disk drives were too expensive to hold data ...