For years, magnetic tape has been one of the most prominent secondary storage alternatives. Although less popular than disk, it is still widely used on all sizes of computer systems. The tapes often are stored either on detachable reels or in cartridges.
Detachable-reel tapes are commonly 5 inch wide, made of plastic Mylar, and coated with a magnetizable substance. A standard reel diameter is 10.5 inches, although smaller, “minireels” are also quite common. A typical 2,400 foot reel can pack data at densities as high as 6,250 bytes per inch.
When such a tape is read, it can transfer more data in one second than many secretaries could type in a month- and error free.
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Cartridge tapes are housed in a small plastic casing. On microcomputer systems, cartridges are commonly used to back up the contents of a hard disk. These tapes can have huge capacities and work very fast.
A .25 inch tape up to 1.000 feet long can hold 10,20,40, or 60 megabytes of data and copy the contents of a disk in a matter of minutes. Recently cartridges using helical-scan recording methods, which store data at an angle to the edge of the tape rather than perpendicular to it, have permitted storage capacities in the 2GB range.
These super-high-capacity tapes can take an hour or longer to back up a hard disk. Cartridge tapes designed specifically for disk backup are sometimes referred to as streaming tapes.
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The IBM 3480 tape storage system, which is used with the IBM 3090 series of mainframe computers, also uses cartridge tapes. These tapes have twice the number of tracks as traditional detachable-reel tapes and a storage capacity of 200 megabytes.
In contrast to streaming tapes, which are intended solely for backup, the IBM 3480 cartridges are designed for general use, including both regular input/output processing operations and backup.
The discussion that follows in this section is based principally on the nine- track, detachable-reel tapes that have been used with mainframes and minicomputers for years. These tape systems are still the most prevalent in the marketplace.
Nonetheless, many of the principles described here apply to cartridge tapes as well.