z/OS® supports many different devices for data storage. Disks or tape are most frequently used for storing data sets on a long-term basis.

Disk drives are known as direct access storage devices (DASDs) because, although some data sets on them might be stored sequentially, these devices can handle direct access. Tape drives are known as sequential access devices because data sets on tape must be accessed sequentially.

The term DASD applies to disks or simulated equivalents of disks. All types of data sets can be stored on DASD (only sequential data sets can be stored on magnetic tape). You use DASD volumes for storing data and executable programs, including the operating system itself, and for temporary working storage. You can use one DASD volume for many different data sets, and reallocate or reuse space on the volume.

To enable the system to locate a specific data set quickly, z/OS includes a data set known as the master catalog that permits access to any of the data sets in the computer system or to other catalogs of data sets. z/OS requires that the master catalog reside on a DASD that is always mounted on a drive that is online to the system.