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Magnetic tape data storage

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Dec 15, 2021

Magnetic tape data storage is a system for storing digital information on magnetic tape using digital recording.

Data storage technologies that use magnetic tape
Computer memory and data storage types
General
Volatile
Historical
Non-volatile

Tape was an important medium for primary data storage in early computers, typically using large open reels of 7-track, later 9-Track tape. Modern magnetic tape is most commonly packaged in cartridges and cassettes, such as the widely supported Linear Tape-Open (LTO)[1] and IBM 3592 series. The device that performs the writing or reading of data is called a tape drive. Autoloaders and tape libraries are often used to automate cartridge handling and exchange. Compatibility was important to enable transferring data.

Tape data storage[2] is now used more for system backup,[3] data archive and data exchange. The low cost of tape has kept it viable for long-term storage and archive.[4]

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Initially, magnetic tape for data storage was wound on 10.5-inch (27 cm)reels.[5] This standard for large computer systems persisted through the late 1980s, with steadily increasing capacity due to thinner substrates and changes in encoding. Tape cartridges and cassettes were available starting in the mid-1970s and were frequently used with small computer systems. With the introduction of the IBM 3480 cartridge in 1984, described as “about one-fourth the size … yet it stored up to 20 percent more data,”[6] large computer systems started to move away from open reel tapes and towards cartridges.[7]

Magnetic tape was first used to record computer data in 1951 on the UNIVAC I. The UNISERVO drive recording medium was a thin metal strip of 0.5-inch (12.7 mm) wide nickel-plated phosphor bronze. Recording density was 128 characters per inch (198 micrometre/character) on eight tracks at a linear speed of 100 in/s (2.54 m/s), yielding a data rate of 12,800 characters per second. Of the eight tracks, six were data, one was for parity, and one was a clock, or timing track. Making allowances for the empty space between tape blocks, the actual transfer rate was around 7,200 characters per second. A small reel of mylar tape provided separation from the metal tape and the read/write head.[8]

10+12-inch (270 mm) diameter reel of 9-track tape

IBM computers from the 1950s used ferric oxide coated tape similar to that used in audio recording. IBM’s technology soon became the de facto industry standard. Magnetic tape dimensions were 0.5-inch (12.7 mm) wide and wound on removable reels. Different tape lengths were available with 1,200 feet (370 m) and 2,400 feet (730 m) on mil and one half thickness being somewhat standard.[clarification needed] During the 1980s, longer tape lengths such as 3,600 feet (1,100 m) became available using a much thinner PET film. Most tape drives could support a maximum reel size of 10.5 inches (267 mm). A so-called mini-reel was common for smaller data sets, such as for software distribution. These were 7-inch (18 cm) reels, often with no fixed lengththe tape was sized to fit the amount of data recorded on it as a cost-saving measure.[citation needed]

CDC used IBM compatible 12-inch (13 mm) magnetic tapes, but also offered a 1-inch-wide (25 mm) variant, with 14 tracks (12 data tracks corresponding to the 12 bit word of CDC 6000 series peripheral processors, plus two parity bits) in the CDC 626 drive.[9]

Early IBM tape drives, such as the IBM 727 and IBM 729, were mechanically sophisticated floor-standing drives that used vacuum columns to buffer long u-shaped loops of tape. Between servo control of powerful reel motors, a low-mass capstan drive, and the low-friction and controlled tension of the vacuum columns, fast start and stop of the tape at the tape-to-head interface could be achieved.[lower-alpha 1] The fast acceleration is possible because the tape mass in the vacuum columns is small; the length of tape buffered in the columns provides time to accelerate the high inertia reels. When active, the two tape reels thus fed tape into or pulled tape out of the vacuum columns, intermittently spinning in rapid, unsynchronized bursts resulting in visually striking action. Stock shots of such vacuum-column tape drives in motion were ironically used to represent computers in movies and television.[10]

Early half-inch tape had seven parallel tracks of data along the length of the tape, allowing six-bit characters plus one bit of parity written across the tape. This was known as seven-track tape. With the introduction of the IBM System/360 mainframe, nine-track tapes were introduced to support the new 8-bit characters that it used. The end of a file was designated by a special recorded pattern called a tape mark, and end of the recorded data on a tape by two successive tape marks. The physical beginning and end of usable tape was indicated by reflective adhesive strips of aluminum foil placed on the backside.[citation needed]

Recording density increased over time. Common seven-track densities started at 200 six-bit characters per inch (CPI), then 556, and finally 800. Nine-track tapes had densities of 800 (using NRZI), then 1600 (using PE), and finally 6250 (using GCR). This translates into about 5 megabytes to 140 megabytes per standard length (2400 ft) reel of tape. Effective density also increased as the interblock gap (inter-record gap) decreased from a nominal 34 inch (19 mm) on seven-track tape reel to a nominal 0.30 inches (7.6 mm) on a 6250 bpi nine track tape reel.[11]

At least partly due to the success of the System/360, and the resultant standardization on 8-bit character codes and byte addressing, nine-track tapes were very widely used throughout the computer industry during the 1970s and 1980s.[12] IBM discontinued new reel-to-reel products replacing them with cartridge based products beginning with its 1984 introduction of the cartridge-based 3480 family.

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