Magnetic tape data storage

Magnetic tape data storage is a system for storing digital information on magnetic tape using digital recording. Modern magnetic tape is most commonly packaged in cartridges and cassettes. The device that performs writing or reading of data is a tape drive. Autoloaders and tape libraries automate cartridge handling. For example, a common cassette-based format is Linear Tape-Open, which comes in a variety of densities and is manufactured by several companies. Magnetic tape data storage is a system for storing digital information on magnetic tape using digital recording. Modern magnetic tape is most commonly packaged in cartridges and cassettes. The device that performs writing or reading of data is a tape drive. Autoloaders and tape libraries automate cartridge handling. For example, a common cassette-based format is Linear Tape-Open, which comes in a variety of densities and is manufactured by several companies. Although magnetic tape was initially primarily for data storage, newer uses included system backup, data archive and data exchange. Initially, magnetic tape for data storage was wound on 10.5-inch (27 cm) reels. This de facto 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,' large computer systems started to move away from open reel tapes and towards cartridges. Magnetic tape was first used to record computer data in 1951 on the Eckert-Mauchly 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 a parity track, 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. IBM computers from the 1950s used ferrous-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 up to 10.5 inches (267 mm) in diameter. 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. 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). CDC used IBM compatible 1/2 inch magnetic tapes, but also offered a 1 inch wide 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. 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 length—the tape was sized to fit the amount of data recorded on it as a cost-saving measure. 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: 1.5 ms from stopped tape to full speed of 112.5 inches per second (2.86 m/s). 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 spin 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 widely used to represent 'the computer' in movies and television. 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. 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. 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 back side.