Compared with magnetic disks in the conventional secondary-storage hierarchy,…

Concept

A storage device is classified along two independent axes: access method and relative access speed.

Direct-access media can reach a selected block without reading every earlier block, whereas sequential-access media must be traversed in order.

Application

An optical disc drive seeks a requested sector with its laser pickup, so its access method is direct rather than sequential. In the conventional hierarchy used by this question, optical drives have longer seek and rotational delays than magnetic disks, so their relative speed is classified as low.

Contrast

• High-speed direct access uses the relevant access method but assigns a higher speed class than the conventional comparison supports.

• Medium-speed direct access uses the relevant access method but assigns an intermediate speed class rather than the conventional low-speed class.

• High-speed sequential access changes both axes: optical discs can seek a selected sector, and their conventional relative speed is not high.

Cross-check and result

A magnetic tape must pass earlier records to reach a later one, which is sequential access. An optical disc can seek the addressed sector, yet its mechanical positioning is comparatively slow; therefore the classification is low-speed direct-access storage device.