Magnetic-tape data storage
Magnetic-tape data storage is a system for storing digital information on magnetic tape using digital recording. Commercial magnetic tape products used for data storage were first released in the 1950s and have continued to be developed and released to the present day.
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 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 is now used more for system backup, data archive and data exchange. The low cost of tape has kept it viable for long-term storage and archive.
Usage
The earliest commercially available computers predate the existence of disk storage. Primary storage for these systems was done using tape. The IBM 701, released in 1952, had the option of a 7-track tape drive, holding over a million characters per reel. Drum storage was also available, but it was much lower capacity, holding around 9 thousand bytes, and it was not interchangeable. Years later and until disks became more affordable, mainframes could still be used with only tape storage, by running TOS/360 and its successors.Looking beyond primary storage, writing data to tape on one computer and then reading it on another has long been a form of data interchange, predating modern data networks and the internet. This form of data transfer has been called Sneakernet. The high-bandwidth, high-latency nature of this is captured by an old, widely repeated quote:
Tape has long been used for making copies of data as part of an orderly backup process. While many other technologies are also used for backups, tape continues to be used for this, particularly at the largest scale.
Similarly, tape continues to see use as an archive for digital preservation efforts. With a low marginal unit cost and long lifespan, tape makes sense for many archiving scenarios.
Open reels
Initially, magnetic tape for data storage was wound on reels. 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", large computer systems started to move away from open-reel tapes and towards cartridges.UNIVAC
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 wide nickel-plated phosphor bronze. Recording density was 128 characters per inch on eight tracks at a linear speed of, 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 between the metal tape and the read/write head.IBM formats
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 wide and wound on removable reels. Different tape lengths were available with and on mil and one half thickness being somewhat standard. During the 1980s, longer tape lengths such as became available using a much thinner PET film. Most tape drives could support a maximum reel size of. A so-called mini-reel was common for smaller data sets, such as for software distribution. These were reels, often with no fixed length—the tape was sized to fit the amount of data recorded on it as a cost-saving measure.CDC used IBM-compatible magnetic tapes, but also offered a variant, with 14 tracks in the CDC 626 drive.
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. 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 emblematically representative of computers in movies and television.
Early half-inch tape had seven parallel tracks of data along the length of the tape, allowing 6-bit characters plus 1 bit of parity written across the tape. This was known as 7-track tape. With the introduction of the IBM System/360 mainframe, 9-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.
Recording density increased over time. Common 7-track densities started at 200 characters per inch, then 556, and finally 800; 9-track tapes had densities of 800, then 1600, and finally 6250. This translates into about 5 megabytes to 140 megabytes per standard length reel of tape. Effective density also increased as the interblock gap decreased from a nominal on 7-track tape reel to a nominal on a 6250 bpi 9-track tape reel.
At least partly due to the success of the System/360, and the resultant standardization on 8-bit character codes and byte addressing, 9-track tapes were very widely used throughout the computer industry during the 1970s and 1980s. IBM discontinued new reel-to-reel products replacing them with cartridge based products beginning with its 1984 introduction of the cartridge-based 3480 family.
DEC format
, and its derivative, DECtape were variations on this "round tape". They were essentially a personal storage medium, used tape that was wide and featured a fixed formatting track which, unlike standard tape, made it feasible to read and rewrite blocks repeatedly in place. LINCtapes and DECtapes had similar capacity and data transfer rate to the diskettes that displaced them, but their access times were on the order of thirty seconds to a minute.Cartridges and cassettes
In the context of magnetic tape, the term cassette or cartridge means a length of magnetic tape in a plastic enclosure with one or two reels for controlling the motion of the tape. The type of packaging affects the load and unload times as well as the length of tape that can be held. In a single-reel cartridge, there is a takeup reel in the drive while a dual reel cartridge has both takeup and supply reels in the cartridge. A tape drive uses one or more precisely controlled motors to wind the tape from one reel to the other, passing a read/write head as it does.File:3590Tape.JPG|right|thumb|An IBM 3590 data cartridge can hold up to 10GiB uncompressed.
A different type is the endless tape cartridge, which has a continuous loop of tape wound on a special reel that allows tape to be withdrawn from the center of the reel and then wrapped up around the edge, and therefore does not need to rewind to repeat. This type is similar to a single-reel cartridge in that there is no take-up reel inside the tape drive.
The IBM 7340 Hypertape drive, introduced in 1961, used a dual reel cassette with a tape capable of holding in excess of 40 million six-bit characters per cassette depending upon record length.
In the 1970s and 1980s, audio Compact Cassettes were frequently used as an inexpensive data storage system for home computers, or in some cases for diagnostics or boot code for larger systems such as the Burroughs B1700. Compact cassettes are logically, as well as physically, sequential; they must be rewound and read from the start to load data. Early cartridges were available before personal computers had affordable disk drives, and could be used as random access devices, automatically winding and positioning the tape, albeit with access times of many seconds.
In 1984 IBM introduced the 3480 family of single reel cartridges and tape drives which were then manufactured by a number of vendors through at least 2004. Initially providing 200 megabytes per cartridge, the family capacity increased over time to 2.4 gigabytes per cartridge. DLT, also a cartridge-based tape, was available beginning 1984 but as of 2007 future development was stopped in favor of LTO.
In 2003 IBM introduced the 3592 family to supersede the IBM 3590. While the name is similar, there is no compatibility between the 3590 and the 3592. Like the 3590 and 3480 before it, this tape format has tape spooled into a single reel cartridge. Initially introduced to support 300 gigabytes, the sixth generation released in 2018 supports a native capacity of 20 terabytes.
Linear Tape-Open single-reel cartridge was announced in 1997 at 100 gigabytes and in its eighth generation supports 12 terabytes in the same sized cartridge. LTO has completely displaced all other tape technologies in computer applications, with the exception of some IBM 3592 family at the high-end.