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Definition: magnetic tape


A sequential storage medium used for data collection, backup and archiving. Magnetic tape is made of flexible plastic with one side coated with a ferromagnetic material. Tapes were originally open reels but were superseded by cartridges and cassettes of many sizes and shapes.

Tape has been more economical than disks for archival data, but that changed as disk capacities increased enormously. In addition, if tapes are stored for the duration, they must be periodically recopied or the tightly coiled magnetic surfaces may contaminate each other. Nevertheless, tape density is expected to surpass magnetic disks and continue well into the future.

Sequential Medium
The major drawback of tape is its sequential format. Locating a specific record requires reading every record in front of it or searching for markers that identify predefined partitions. Although most tapes are used for archiving rather than routine updating, some drives allow rewriting in place if the byte count does not change. Otherwise, updating requires reading the original tape, changing the data and rewriting everything onto another tape. New records are inserted at the appropriate point.

Track Formats
Tracks run parallel to the edge of the tape (linear recording) or diagonally (helical scan). A linear variation is serpentine recording, in which the tracks "snake" back and forth from the end of the tape to the beginning.

Legacy open reel tapes used nine linear tracks (8 bits plus parity), while modern cartridges use 128 or more tracks. Data are recorded in blocks of contiguous bytes, separated by a space called an "interrecord gap" or "interblock gap." Tape drive speed is measured in inches per second (ips). Over the decades, storage density jumped from 200 bits per square inch to millions and billions of bits. See helical scan and compact tape. See also magnetic disk and optical disc.




Tracks on Magnetic Tape
Except for helical scan recording, most tracks on magnetic tape run parallel to the length of the tape.




Magnetic Tape Summary
LTO is the only tape format that is still widely used. See LTO.






Old Tape Formats
The following tape technologies are no longer used. In order to read the data stored in these formats, the tapes must be converted to LTO, hard drives or solid state drives as the drives that read them are increasingly hard to find. In addition, drivers for the tape hardware may also not work in current versions of operating systems.