Magnetic tape data storage
Encyclopedia
Magnetic tape data storage uses digital recording
on to magnetic tape
to store digital information. Modern magnetic tape is most commonly packaged in cartridge
s and cassettes
. The device that performs actual writing or reading of data is a tape drive
. Autoloaders and tape libraries
are frequently used to automate cartridge handling.
When storing large amounts of data, tape can be substantially less expensive than disk or other data storage options. Tape storage has always been used with large computer systems. Modern usage is primarily as a high capacity medium for backup
s and archive
s. As of 2011, the highest capacity tape cartridges (T10000C
) can store 5 TB
of uncompressed data.
Initially, magnetic tape for data storage was wound on large (10.5 in/26.67 cm) reel
s. This defacto standard for large computer systems persisted through the late 1980s. Tape cartridges and cassettes were available as early as the mid 1970s and were frequently used with small computer systems. With the introduction of the IBM 3480 cartridge in 1984, large computer systems started to move away from open reel tapes and towards cartridges.
-Mauchly
UNIVAC I
. The UNISERVO
drive recording medium was a thin metal strip of ½″ wide(12.7 mm) 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 allowance 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.
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" (12.7 mm) wide and wound on removable reels of up to 10.5 inches (267 mm) in diameter. Different tape lengths were available with 1200', 2400' on mil and one half thickness being somewhat standard. Later during the '80s, longer tape lengths such as 3600' became available, but only with a much thinner PET film
. Most tape drives could support a maximum reel size of 10.5"
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 active control of powerful reel motors and vacuum control of these u-shaped tape loops, extremely rapid start and stop of the tape at the tape-to-head interface could be achieved. (1.5ms from stopped tape to full speed of up to 112.5 IPS) 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 7 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 7-track tape
. With the introduction of the IBM System 360 mainframe, 9 track tapes were developed to support the new 8-bit characters that it used. Effective recording density increased over time. Common 7-track densities started at 200, then 556, and finally 800 cpi and 9-track tapes had densities of 800, 1600, and 6250 cpi. This translates into about 5 MB to 140 MB per standard length (2400 ft) reel of tape. End of file was designated by a tape mark and end of tape by two tape marks.
At least partly due to the success of the S/360, 9-track tapes were very widely used throughout the industry during the 1970s and 1980s.
, were variations on this "round tape." They were essentially a personal storage medium. The tape was ¾ inch 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 "seek times" were on the order of thirty seconds to a minute.IRG of Magnetic tape is 0.75 inch
In the context of magnetic tape, the term cassette usually refers to an enclosure that holds two reels with a single span of magnetic tape. The term cartridge is more generic, but frequently means a single reel of tape in a plastic enclosure.
The type of packaging is a large determinant of the load and unload times as well as the length of tape that can be held. A tape drive that uses a single reel cartridge has a takeup reel in the drive while cassettes have the take up reel in the cassette. A tape drive (or "transport" or "deck") uses precisely controlled motors to wind the tape from one reel to the other, passing a read/write head as it does.
A different type of tape cartridge 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. This type is similar to a cassette in that there is no take-up reel inside the tape drive.
In the 1970s and 1980s, audio Compact Cassettes were frequently used as an inexpensive data storage system for home computers. Compact cassettes were logically, as well as physically, sequential; they had to 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.
Most modern magnetic tape systems use reels that are fixed inside a cartridge to protect the tape and facilitate handling. Modern cartridge formats include DDS/DAT
, DLT
and LTO
with capacities in the tens to hundreds of gigabytes.
Recording method is also an important way to classify tape technologies, generally falling into two categories:
A variation on linear technology is linear serpentine recording, which uses more tracks than tape heads. Each head still writes one track at a time. After making a pass over the whole length of the tape, all heads shift slightly and make another pass in the reverse direction, writing another set of tracks. This procedure is repeated until all tracks have been read or written. By using the linear serpentine method, the tape medium can have many more tracks than read/write heads. Compared to simple linear recording, using the same tape length and the same number of heads, the data storage capacity is substantially higher.
Scanning recording methods write short dense tracks across the width of the tape medium, not along the length. Tape heads are placed on a drum or disk which rapidly rotates while the relatively slowly moving tape passes it.
An early method used to get a higher data rate than the prevailing linear method was transverse scan. In this method a spinning disk, with the tape heads embedded in the outer edge, is placed perpendicular to the path of the tape. This method is used in Ampex
's DCRsi instrumentation data recorders and the old Ampex
2 inch Quadruplex videotape
system. Another early method was arcuate scan. In this method, the heads are on the face of a spinning disk which is laid flat against the tape. The path of the tape heads makes an arc.
Helical scan
recording writes short dense tracks in diagonal manner. This recording method is used by virtually all videotape
systems and several data tape formats.
Various methods have been used alone and in combination to cope with this difference. The tape drive can be stopped, backed up, and restarted (known as shoe-shining, because of increased wear of both medium and head). A large memory buffer can be used to queue the data. The host can assist this process by choosing appropriate block sizes to send to the tape drive. There is a complex tradeoff between block size, the size of the data buffer in the record/playback deck, the percentage of tape lost on inter-block gaps, and read/write throughput.
Finally modern tape drives offer speed matching feature, where drive can dynamically decrease physical tape speed as much as 50% to avoid shoe-shining.
to data. While tape can provide a very high data transfer rate for streaming long contiguous sequences of data, it takes in the 10's of seconds to reposition the tape head to an arbitrarily chosen place on the tape. In contrast, hard disk technology can perform the equivalent action in 10's of milliseconds (3 orders of magnitude faster) and can be thought of as offering random access
to data.
Logical filesystems require data and metadata to be stored on the data storage medium. Storing metadata in one place and data in another requires lots of slow repositioning activity on most tape systems. As a result, most tape systems use a very trivial filesystem in which files are addressed by number not by filename. Metadata
such as file name or modification time is typically not stored at all. Tape label
s store such metadata, and they are used for interchanging data between systems. File archiver
and backup
tools have been created to pack multiple files along with the related metadata into a single 'tape file'. Serpentine tape drives (e.g. QIC) can improve access time by switching to the appropriate track; tape partitions were used for directory information. The Linear Tape File System is a method of storing file metadata on a separate part of the tape. This makes it possible to copy and paste files or directories to a tape as if it were just like another disk, but does not change the fundamental sequential access nature of tape.
. There are several algorithms which provide similar results: LZ (most), IDRC (Exabyte), ALDC (IBM, QIC) and DLZ1 (DLT). Embedded in tape drive hardware, these compress a relatively small buffer of data at a time, so cannot achieve extremely high compression even of highly redundant data. A ratio of 2:1 is typical, with some vendors claiming 2.6:1 or 3:1. The ratio actually obtained with real data is often less than the stated figure; the compression ratio cannot be relied upon when specifying the capacity of equipment, e.g., a drive claiming a compressed capacity of 500GB may not be adequate to back up 500GB of real data. Data that is already stored efficiently may not allow any significant compression; a sparse database may offer much larger factors. Software compression can achieve much better results with sparse data, but uses the host computer's processor, and can slow the backup if it is unable to compress as fast as the data is written.
Some enterprise tape drives can encrypt
data (this must be done after compression, as encrypted data cannot be compressed effectively). Symmetric streaming encryption algorithms are also implemented to provide high performance.
The compression algorithms used in low-end products are not the most effective known today, and better results can usually be obtained by turning off hardware compression, using software compression (and encryption if desired) instead.
The costs of disk storage have decreased faster than that of tapes. Until about the end of the twentieth century prices and capacities allowed backing up a desktop hard drive to tape, such as inexpensive Travan
, much more cheaply and more compactly than backing up to an additional, external or removable, drive. Later drive prices dropped, drives with capacities of hundreds to a few thousands of megabytes started to be used on relatively inexpensive machines, and backing up to an external USB drive became cheaper, and the drive more compact, than tape for a non-networked machine used by a business or serious user.
As a basic comparison, mainframe
-class tape drives, such as Oracle's Sun StorageTek T10000B, are priced at approximately US$37,000 each, excluding tape libraries. (IBM's TS1130 is also representative of this storage class.) At any single moment in time each T10000C tape drive can read and/or write to one tape cartridge which can contain up to 5TB of uncompressed data. Real-world sequential data transfer speeds are high (sustained 240MB/second for the T10000C and 160MB/second for the TS1130) compared to disk. However, PC-class hard disks are priced below $200 for about 3TB. One mainframe-class hard disk still has a much lower price than one mainframe-class tape drive, so the economics might favor disk.
However, the key difference is that tape drives can exchange their magnetic media (the cartridges) frequently, while the magnetic media installed inside each hard disk is fixed and cannot be swapped. (The drives themselves could be moved if installed in swappable caddies at extra cost, with extra cost hot-swappable infrastructure.) Mainframe-class tape drives are almost always installed in robotic tape libraries which are often quite large and can hold thousands of cartridges. The StorageTek SL8500 library is one representative example (and IBM also sells tape libraries). The smallest SL8500 library holds up to 1,448 tape cartridges, for 1.4 Petabytes of online uncompressed storage. An equivalent amount of PC-class hard disk storage would be priced at $100,000 or more for the drives. The tape library would likely deliver a higher sustained sequential write speed, the media would be more rugged (for off-site storage), the media would meet or exceed long-term archival storage requirements (for reliable retrieval decades into the future), and the data center power and cooling requirements would be considerably lower. The economics of this comparison are more complicated than a single-spindle versus tape drive comparison.
Whether tape's characteristics versus disk are useful or not will depend on the particular data center and its data storage requirements. What has tended to happen in recent years is that the amount of data has grown exponentially, with both disk (especially) and tape participating in the growth. In the early twentyfirst century solid state storage encroached on disk's previous near-monopoly in random access non-volatile data storage, while disk pushed into tape's territory to some extent, particularly in situations where sequential data access is only a relatively small part of a particular data center's storage requirements.
These are just lists, not the actual standards.
Digital recording
In digital recording, digital audio and digital video is directly recorded to a storage device as a stream of discrete numbers, representing the changes in air pressure for audio and chroma and luminance values for video through time, thus making an abstract template for the original sound or...
on to magnetic tape
Magnetic tape
Magnetic tape is a medium for magnetic recording, made of a thin magnetizable coating on a long, narrow strip of plastic. It was developed in Germany, based on magnetic wire recording. Devices that record and play back audio and video using magnetic tape are tape recorders and video tape recorders...
to store digital information. Modern magnetic tape is most commonly packaged in cartridge
Cartridge
Cartridge may refer to:* 8-track tape cartridge, a 1960s-1980s music storage format* Broadcast cartridge used in radio stations* Cartridge , a round of ammunition* Cartridge...
s and cassettes
Compact Cassette
The Compact Cassette, often referred to as audio cassette, cassette tape, cassette, or simply tape, is a magnetic tape sound recording format. It was designed originally for dictation, but improvements in fidelity led the Compact Cassette to supplant the Stereo 8-track cartridge and reel-to-reel...
. The device that performs actual writing or reading of data is a tape drive
Tape drive
A tape drive is a data storage device that reads and performs digital recording, writes data on a magnetic tape. Magnetic tape data storage is typically used for offline, archival data storage. Tape media generally has a favorable unit cost and long archival stability.A tape drive provides...
. Autoloaders and tape libraries
Tape library
In computer storage, a tape library, sometimes called a tape silo, tape robot or tape jukebox, is a storage device which contains one or more tape drives, a number of slots to hold tape cartridges, a barcode reader to identify tape cartridges and an automated method for loading tapes...
are frequently used to automate cartridge handling.
When storing large amounts of data, tape can be substantially less expensive than disk or other data storage options. Tape storage has always been used with large computer systems. Modern usage is primarily as a high capacity medium for backup
Backup
In information technology, a backup or the process of backing up is making copies of data which may be used to restore the original after a data loss event. The verb form is back up in two words, whereas the noun is backup....
s and archive
Archive
An archive is a collection of historical records, or the physical place they are located. Archives contain primary source documents that have accumulated over the course of an individual or organization's lifetime, and are kept to show the function of an organization...
s. As of 2011, the highest capacity tape cartridges (T10000C
StorageTek tape formats
Storage Technology Corporation, or StorageTek, has created several magnetic tape data storage formats. These are commonly used with large computer systems, typically in conjunction with a robotic tape library. The most recent format is the T10000...
) can store 5 TB
Terabyte
The terabyte is a multiple of the unit byte for digital information. The prefix tera means 1012 in the International System of Units , and therefore 1 terabyte is , or 1 trillion bytes, or 1000 gigabytes. 1 terabyte in binary prefixes is 0.9095 tebibytes, or 931.32 gibibytes...
of uncompressed data.
Open reels
Reel
A reel is an object around which lengths of another material are wound for storage. Generally a reel has a cylindrical core and walls on the sides to retain the material wound around the core...
s. This defacto standard for large computer systems persisted through the late 1980s. Tape cartridges and cassettes were available as early as the mid 1970s and were frequently used with small computer systems. With the introduction of the IBM 3480 cartridge in 1984, 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 EckertJ. Presper Eckert
John Adam Presper "Pres" Eckert Jr. was an American electrical engineer and computer pioneer. With John Mauchly he invented the first general-purpose electronic digital computer , presented the first course in computing topics , founded the first commercial computer company , and...
-Mauchly
John Mauchly
John William Mauchly was an American physicist who, along with J. Presper Eckert, designed ENIAC, the first general purpose electronic digital computer, as well as EDVAC, BINAC and UNIVAC I, the first commercial computer made in the United States.Together they started the first computer company,...
UNIVAC I
UNIVAC I
The UNIVAC I was the first commercial computer produced in the United States. It was designed principally by J. Presper Eckert and John Mauchly, the inventors of the ENIAC...
. The UNISERVO
UNISERVO
The UNISERVO tape drive was the primary I/O device on the UNIVAC I computer. Its place in history is assured as it was the first tape drive for a commercially sold computer....
drive recording medium was a thin metal strip of ½″ wide(12.7 mm) nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
-plated phosphor bronze
Phosphor bronze
Phosphor bronze is an alloy of copper with 3.5 to 10% of tin and a significant phosphorus content of up to 1%. The phosphorus is added as deoxidizing agent during melting....
. 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
Parity bit
A parity bit is a bit that is added to ensure that the number of bits with the value one in a set of bits is even or odd. Parity bits are used as the simplest form of error detecting code....
, and one was a clock, or timing track. Making allowance 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 formats
IBM computers from the 1950sIBM 700/7000 series
The IBM 700/7000 series was a series of large-scale computer systems made by IBM through the 1950s and early 1960s. The series included several different, incompatible processor architectures. The 700s used vacuum tube logic and were made obsolete by the introduction of the transistorized 7000s...
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" (12.7 mm) wide and wound on removable reels of up to 10.5 inches (267 mm) in diameter. Different tape lengths were available with 1200', 2400' on mil and one half thickness being somewhat standard. Later during the '80s, longer tape lengths such as 3600' became available, but only with a much thinner PET film
PET film (biaxially oriented)
BoPET is a polyester film made from stretched polyethylene terephthalate and is used for its high tensile strength, chemical and dimensional stability, transparency, reflectivity, gas and aroma barrier properties and electrical insulation.A variety of companies manufacture boPET and other...
. Most tape drives could support a maximum reel size of 10.5"
Early IBM tape drives, such as the IBM 727
IBM 727
The IBM 727 Magnetic Tape Unit was announced for the IBM 701 and IBM 702 on September 25, 1953. It became IBM's standard tape drive for their early vacuum tube era computer systems. Later vacuum tube machines and first-generation transistor computers used the IBM 729-series tape drive...
and IBM 729
IBM 729
The IBM 729 Magnetic Tape Unit was IBM's iconic tape mass storage system from the late 1950s through the mid 1960s. Part of the IBM 7 track family of tape units, it was used on late 700, most 7000 and many 1400 series computers...
, were mechanically sophisticated floor-standing drives that used vacuum columns to buffer long u-shaped loops of tape. Between active control of powerful reel motors and vacuum control of these u-shaped tape loops, extremely rapid start and stop of the tape at the tape-to-head interface could be achieved. (1.5ms from stopped tape to full speed of up to 112.5 IPS) 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 7 parallel tracks of data along the length of the tape allowing, six-bit characters plus one bit of parity
Parity bit
A parity bit is a bit that is added to ensure that the number of bits with the value one in a set of bits is even or odd. Parity bits are used as the simplest form of error detecting code....
written across the tape. This was known as 7-track tape
IBM 7 Track
IBM's first magnetic tape data storage devices, introduced in 1952, use what is now generally known as 7 track tape. The magnetic tape is 1/2" wide and there are 6 data tracks plus 1 parity track for a total of 7 parallel tracks that span the length of the tape...
. With the introduction of the IBM System 360 mainframe, 9 track tapes were developed to support the new 8-bit characters that it used. Effective recording density increased over time. Common 7-track densities started at 200, then 556, and finally 800 cpi and 9-track tapes had densities of 800, 1600, and 6250 cpi. This translates into about 5 MB to 140 MB per standard length (2400 ft) reel of tape. End of file was designated by a tape mark and end of tape by two tape marks.
At least partly due to the success of the S/360, 9-track tapes were very widely used throughout the industry during the 1970s and 1980s.
DEC format
LINCtape, and its derivative, DECtapeDECtape
DECtape, originally called "Microtape", was a magnetic tape data storage medium used with many Digital Equipment Corporation computers, including the PDP-6, PDP-8, LINC-8, PDP-10, PDP-11, PDP-12, and the PDP-15. On DEC's 32-bit systems, VAX/VMS support for it was implemented but did not become an...
, were variations on this "round tape." They were essentially a personal storage medium. The tape was ¾ inch 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 "seek times" were on the order of thirty seconds to a minute.IRG of Magnetic tape is 0.75 inch
Cartridges and cassettes
The type of packaging is a large determinant of the load and unload times as well as the length of tape that can be held. A tape drive that uses a single reel cartridge has a takeup reel in the drive while cassettes have the take up reel in the cassette. A tape drive (or "transport" or "deck") uses precisely controlled motors to wind the tape from one reel to the other, passing a read/write head as it does.
A different type of tape cartridge 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. This type is similar to a cassette in that there is no take-up reel inside the tape drive.
In the 1970s and 1980s, audio Compact Cassettes were frequently used as an inexpensive data storage system for home computers. Compact cassettes were logically, as well as physically, sequential; they had to 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
Random access
In computer science, random access is the ability to access an element at an arbitrary position in a sequence in equal time, independent of sequence size. The position is arbitrary in the sense that it is unpredictable, thus the use of the term "random" in "random access"...
devices, automatically winding and positioning the tape, albeit with access times of many seconds.
Most modern magnetic tape systems use reels that are fixed inside a cartridge to protect the tape and facilitate handling. Modern cartridge formats include DDS/DAT
Digital Data Storage
Digital Data Storage is a format for storing computer data on a Digital Audio Tape .DDS uses tape with a width of 3.8mm, with the exception of the latest formats, DAT 160 and DAT 320, which are 8mm wide...
, DLT
Digital Linear Tape
Digital Linear Tape is a magnetic tape data storage technology developed by Digital Equipment Corporation from 1984 onwards. In 1994 the technology was purchased by Quantum Corporation, who currently manufactures drives and licenses the technology and trademark. A variant with higher capacity...
and LTO
Linear Tape-Open
Linear Tape-Open is a magnetic tape data storage technology originally developed in the late 1990s as an open standards alternative to the proprietary magnetic tape formats that were available at the time. Hewlett-Packard, IBM and Seagate initiated the LTO Consortium, which directs development...
with capacities in the tens to hundreds of gigabytes.
Tape width
Medium width is the primary classification criterion for tape technologies. Half inch has historically been the most common width of tape for high capacity data storage. Many other sizes exist and most were developed to either have smaller packaging or higher capacity.Recording method
Linear
The linear method arranges data in long parallel tracks that span the length of the tape. Multiple tape heads simultaneously write parallel tape tracks on a single medium. This method was used in early tape drives. It is the simplest recording method, but has the lowest data density.Scanning
An early method used to get a higher data rate than the prevailing linear method was transverse scan. In this method a spinning disk, with the tape heads embedded in the outer edge, is placed perpendicular to the path of the tape. This method is used in Ampex
Ampex
Ampex is an American electronics company founded in 1944 by Alexander M. Poniatoff. The name AMPEX is an acronym, created by its founder, which stands for Alexander M. Poniatoff Excellence...
's DCRsi instrumentation data recorders and the old Ampex
Ampex
Ampex is an American electronics company founded in 1944 by Alexander M. Poniatoff. The name AMPEX is an acronym, created by its founder, which stands for Alexander M. Poniatoff Excellence...
2 inch Quadruplex videotape
2 inch Quadruplex videotape
2-inch quadruplex videotape was the first practical and commercially successful analog recording videotape format. It was developed and released for the broadcast television industry in 1956 by Ampex, an American company based in Redwood City, California...
system. Another early method was arcuate scan. In this method, the heads are on the face of a spinning disk which is laid flat against the tape. The path of the tape heads makes an arc.
Helical scan
Helical scan
Helical scan is a method of recording high bandwidth signals onto magnetic tape. It is used in reel-to-reel video tape recorders, video cassette recorders, digital audio tape recorders, and some computer tape drives....
recording writes short dense tracks in diagonal manner. This recording method is used by virtually all videotape
Videotape
A videotape is a recording of images and sounds on to magnetic tape as opposed to film stock or random access digital media. Videotapes are also used for storing scientific or medical data, such as the data produced by an electrocardiogram...
systems and several data tape formats.
Block layout
In a typical format, data is written to tape in blocks with inter-block gaps between them, and each block is written in a single operation with the tape running continuously during the write. However, since the rate at which data is written or read to the tape drive is not deterministic, a tape drive usually has to cope with a difference between the rate at which data goes on and off the tape and the rate at which data is supplied or demanded by its host.Various methods have been used alone and in combination to cope with this difference. The tape drive can be stopped, backed up, and restarted (known as shoe-shining, because of increased wear of both medium and head). A large memory buffer can be used to queue the data. The host can assist this process by choosing appropriate block sizes to send to the tape drive. There is a complex tradeoff between block size, the size of the data buffer in the record/playback deck, the percentage of tape lost on inter-block gaps, and read/write throughput.
Finally modern tape drives offer speed matching feature, where drive can dynamically decrease physical tape speed as much as 50% to avoid shoe-shining.
Sequential access to data
Tape is characterized by sequential accessSequential access
In computer science, sequential access means that a group of elements is accessed in a predetermined, ordered sequence. Sequential access is sometimes the only way of accessing the data, for example if it is on a tape...
to data. While tape can provide a very high data transfer rate for streaming long contiguous sequences of data, it takes in the 10's of seconds to reposition the tape head to an arbitrarily chosen place on the tape. In contrast, hard disk technology can perform the equivalent action in 10's of milliseconds (3 orders of magnitude faster) and can be thought of as offering random access
Random access
In computer science, random access is the ability to access an element at an arbitrary position in a sequence in equal time, independent of sequence size. The position is arbitrary in the sense that it is unpredictable, thus the use of the term "random" in "random access"...
to data.
Logical filesystems require data and metadata to be stored on the data storage medium. Storing metadata in one place and data in another requires lots of slow repositioning activity on most tape systems. As a result, most tape systems use a very trivial filesystem in which files are addressed by number not by filename. Metadata
Metadata
The term metadata is an ambiguous term which is used for two fundamentally different concepts . Although the expression "data about data" is often used, it does not apply to both in the same way. Structural metadata, the design and specification of data structures, cannot be about data, because at...
such as file name or modification time is typically not stored at all. Tape label
Tape label
Tape labels are identifiers given to volumes of magnetic tape.There are two kinds of tape labels. The first is a label applied to the exterior of tape cartridge or reel. The second is data recorded on the tape itself.-Visual labels:...
s store such metadata, and they are used for interchanging data between systems. File archiver
File archiver
A file archiver is a computer program that combines a number of files together into one archive file, or a series of archive files, for easier transportation or storage...
and backup
Backup
In information technology, a backup or the process of backing up is making copies of data which may be used to restore the original after a data loss event. The verb form is back up in two words, whereas the noun is backup....
tools have been created to pack multiple files along with the related metadata into a single 'tape file'. Serpentine tape drives (e.g. QIC) can improve access time by switching to the appropriate track; tape partitions were used for directory information. The Linear Tape File System is a method of storing file metadata on a separate part of the tape. This makes it possible to copy and paste files or directories to a tape as if it were just like another disk, but does not change the fundamental sequential access nature of tape.
Access time
Tape has quite a long latency for random accesses since the deck must wind an average of one-third the tape length to move from one arbitrary data block to another. Most tape systems attempt to alleviate the intrinsic long latency, either using indexing, where a separate lookup table (tape directory) is maintained which gives the physical tape location for a given data block number (a must for serpentine drives), by marking blocks with a tape mark that can be detected while winding the tape at high speed.Data compression
Most tape drives now include some kind of data compressionData compression
In computer science and information theory, data compression, source coding or bit-rate reduction is the process of encoding information using fewer bits than the original representation would use....
. There are several algorithms which provide similar results: LZ (most), IDRC (Exabyte), ALDC (IBM, QIC) and DLZ1 (DLT). Embedded in tape drive hardware, these compress a relatively small buffer of data at a time, so cannot achieve extremely high compression even of highly redundant data. A ratio of 2:1 is typical, with some vendors claiming 2.6:1 or 3:1. The ratio actually obtained with real data is often less than the stated figure; the compression ratio cannot be relied upon when specifying the capacity of equipment, e.g., a drive claiming a compressed capacity of 500GB may not be adequate to back up 500GB of real data. Data that is already stored efficiently may not allow any significant compression; a sparse database may offer much larger factors. Software compression can achieve much better results with sparse data, but uses the host computer's processor, and can slow the backup if it is unable to compress as fast as the data is written.
Some enterprise tape drives can encrypt
Encryption
In cryptography, encryption is the process of transforming information using an algorithm to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information...
data (this must be done after compression, as encrypted data cannot be compressed effectively). Symmetric streaming encryption algorithms are also implemented to provide high performance.
The compression algorithms used in low-end products are not the most effective known today, and better results can usually be obtained by turning off hardware compression, using software compression (and encryption if desired) instead.
Viability
For decades tape storage has offered cost and storage density advantages over many other storage technologies, such as disk storage. And for decades medium-sized and large-sized data centers have deployed both tape and disk storage to complement each other, with tape the favorite choice for tertiary and archival data storage. Storage technologies continue to advance both functionally and economically, and storage vendors compete aggressively against each other. Analysts are lining up on both sides of the "disk versus tape" argument.The costs of disk storage have decreased faster than that of tapes. Until about the end of the twentieth century prices and capacities allowed backing up a desktop hard drive to tape, such as inexpensive Travan
Travan
Travan is an 8 mm magnetic tape cartridge design developed by the 3M company, used for the storage of data in computer backups and mass storage. Over time, subsequent versions of Travan cartridges and drives have been developed that provide greater data capacity, while retaining the standard 8 mm...
, much more cheaply and more compactly than backing up to an additional, external or removable, drive. Later drive prices dropped, drives with capacities of hundreds to a few thousands of megabytes started to be used on relatively inexpensive machines, and backing up to an external USB drive became cheaper, and the drive more compact, than tape for a non-networked machine used by a business or serious user.
As a basic comparison, mainframe
Mainframe computer
Mainframes are powerful computers used primarily by corporate and governmental organizations for critical applications, bulk data processing such as census, industry and consumer statistics, enterprise resource planning, and financial transaction processing.The term originally referred to the...
-class tape drives, such as Oracle's Sun StorageTek T10000B, are priced at approximately US$37,000 each, excluding tape libraries. (IBM's TS1130 is also representative of this storage class.) At any single moment in time each T10000C tape drive can read and/or write to one tape cartridge which can contain up to 5TB of uncompressed data. Real-world sequential data transfer speeds are high (sustained 240MB/second for the T10000C and 160MB/second for the TS1130) compared to disk. However, PC-class hard disks are priced below $200 for about 3TB. One mainframe-class hard disk still has a much lower price than one mainframe-class tape drive, so the economics might favor disk.
However, the key difference is that tape drives can exchange their magnetic media (the cartridges) frequently, while the magnetic media installed inside each hard disk is fixed and cannot be swapped. (The drives themselves could be moved if installed in swappable caddies at extra cost, with extra cost hot-swappable infrastructure.) Mainframe-class tape drives are almost always installed in robotic tape libraries which are often quite large and can hold thousands of cartridges. The StorageTek SL8500 library is one representative example (and IBM also sells tape libraries). The smallest SL8500 library holds up to 1,448 tape cartridges, for 1.4 Petabytes of online uncompressed storage. An equivalent amount of PC-class hard disk storage would be priced at $100,000 or more for the drives. The tape library would likely deliver a higher sustained sequential write speed, the media would be more rugged (for off-site storage), the media would meet or exceed long-term archival storage requirements (for reliable retrieval decades into the future), and the data center power and cooling requirements would be considerably lower. The economics of this comparison are more complicated than a single-spindle versus tape drive comparison.
Whether tape's characteristics versus disk are useful or not will depend on the particular data center and its data storage requirements. What has tended to happen in recent years is that the amount of data has grown exponentially, with both disk (especially) and tape participating in the growth. In the early twentyfirst century solid state storage encroached on disk's previous near-monopoly in random access non-volatile data storage, while disk pushed into tape's territory to some extent, particularly in situations where sequential data access is only a relatively small part of a particular data center's storage requirements.
Chronological list of tape formats
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Scalable Linear Recording Scalable Linear Recording is the name used by Tandberg Data for its line of QIC based tape drives.The earliest SLR drive, the SLR1, has a capacity of 250 MB, while the latest drive, the SLR140, has a capacity of 70 GB... Digital Data Storage Digital Data Storage is a format for storing computer data on a Digital Audio Tape .DDS uses tape with a width of 3.8mm, with the exception of the latest formats, DAT 160 and DAT 320, which are 8mm wide... (DDS) on Digital Audio Tape Digital Audio Tape Digital Audio Tape is a signal recording and playback medium developed by Sony and introduced in 1987. In appearance it is similar to a compact audio cassette, using 4 mm magnetic tape enclosed in a protective shell, but is roughly half the size at 73 mm × 54 mm × 10.5 mm. As... (DAT)
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See also
- Computer data storage
- Magnetic storageMagnetic storageMagnetic storage and magnetic recording are terms from engineering referring to the storage of data on a magnetized medium. Magnetic storage uses different patterns of magnetization in a magnetizable material to store data and is a form of non-volatile memory. The information is accessed using...
- Tape driveTape driveA tape drive is a data storage device that reads and performs digital recording, writes data on a magnetic tape. Magnetic tape data storage is typically used for offline, archival data storage. Tape media generally has a favorable unit cost and long archival stability.A tape drive provides...
- Information repositoryInformation repositoryAn information repository is an easy way to deploy a secondary tier of data storage that can comprise multiple, networked data storage technologies running on diverse operating systems, where data that no longer needs to be in primary storage is protected, classified according to captured metadata,...
- Data proliferationData proliferationData proliferation refers to the prodigious amount of data, structured and unstructured, that businesses and governments continue to generate at an unprecedented rate and the usability problems that result from attempting to store and manage that data...
- Tape mark
ISO Standard lists
These are just lists, not the actual standards.