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Optical disk library systems are designed to bring data normally stored on microfiche or paper to an online device where it is quickly accessible by network users. An optical disk library can also supplement magnetic tape backup systems or serve as an intermediate storage device for data that is "migrating" to magnetic tape in an HMS (hierarchical management system).

In an HMS system, little-used files or files that have been marked for migration are moved from magnetic disk to optical disk, where they remain available to users. In this scheme, the optical disk is called near-line storage, as shown in Figure O-2. Eventually, they are moved to magnetic tape for archiving purposes. While magnetic tape offers a convenient, removable, and economical storage media, its sequential access method makes it unsuitable for online data retrieval.