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Optical Libraries

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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.

Figure 1 (see book)

Hierarchical systems will move files on optical disk to magnetic disk when requested by a user. Users are often unaware that the files are from an optical disk, except for a slight delay in accessing the files while they are moved from near-line to online storage. A hierarchical management system keeps a copy of the directory for near-line files available online to users.

This topic is covered further under "Storage Management Systems."

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