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Tape drive
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A tape drive, which is also known as a streamer, is a data storage device that reads and writes data stored on a magnetic tape. It is typically used for archival storage of data stored on hard drives. Tape media generally has a favorable unit cost and long archival stability.
Instead of allowing random-access to data as hard disk drives do, tape drives only allow for sequential-access of data.
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Encyclopedia
A tape drive, which is also known as a streamer, is a data storage device that reads and writes data stored on a magnetic tape. It is typically used for archival storage of data stored on hard drives. Tape media generally has a favorable unit cost and long archival stability.
Instead of allowing random-access to data as hard disk drives do, tape drives only allow for sequential-access of data. A hard disk drive can move its read/write heads to any random part of the disk platters in a very short amount of time, but a tape drive must spend a considerable amount of time winding tape between reels to read any one particular piece of data. As a result, tape drives have very slow average seek times. Despite the slow seek time, tape drives can stream data to tape very quickly. For example, modern LTO drives can reach continuous data transfer rates of up to 80 MB/s, which is as fast as most 10,000 rpm hard disks.
Tape drives can range in capacity from a few megabytes to hundreds of gigabytes, uncompressed. In marketing materials, tape storage is usually referred to with the assumption of 2:1 compression ratio, so a tape drive might be known as 80/160, meaning that the true storage capacity is 80 whilst the compressed storage capacity can be approximately 160 in many situations. IBM and Sony have also used higher compression ratios in their marketing materials. The real-world, observed compression ratio always depends on what type of data is being compressed. The true storage capacity is also known as the native capacity or the raw capacity.
Tape drives can be connected to a computer with SCSI (most common), Fibre Channel, SATA, USB, FireWire, FICON, or other interfaces. Tape drives can be found inside autoloaders and tape libraries which assist in loading, unloading and storing multiple tapes to further increase archive capacity.
Some older tape drives were designed as inexpensive alternatives to disk drives. Examples include DECtape, the ZX Microdrive and Rotronics Wafadrive. This is generally not feasible with modern tape drives that use advanced techniques like multilevel forward error correction, shingling, and serpentine layout for writing data to tape.
Shoe-shining effect
The shoe-shining effect occurs during writing or reading data to tape, when the transfer rate of the data falls below the minimum threshold at which the tape drive heads were designed to transfer data to a running tape. When this occurs, the drive must decelerate the tape, stop it, rewind back a little, accelerate again to a proper speed and continue writing from the same position.
In early drives, such start-stop work was often unavoidable. Early tape drives used very large tape spools which necessarily had high inertia and did not start and stop moving easily. To provide high tape-seeking performance, several feet of loose tape was played out and pulled by a suction fan down into two deep open channels on either side of the tape head and capstans. The long thin loops of tape hanging in these vacuum columns had far less inertia than the two reels and could be rapidly repositioned. The large reels would occasionally move to take up written tape and play out more blank tape into the vacuum columns.
Later, most tape drive designs of the 1980s introduced the internal data buffer to somewhat reduce start-stop situations. The tape was stopped only when the buffer contained no data to be written (buffer underrun), or when it was full of data during reading.
Most recently, drives no longer operate at single fixed linear speed, but have a few speed levels. Internally, they implement algorithms that dynamically match the tape speed level to computer's data rate. Example speed levels could be 50%, 75% and 100% of full speed. Still, a computer that streams data constantly below the lowest speed level (e.g. at 49%) will undoubtedly cause shoe-shining.
When shoe-shining occurs, it significantly affects the attainable data rate. It is most important in backup process to modern fast drives. Furthermore, shoe-shining places undue stress on the drive mechanism and the tape medium itself, increasing hardware failure rate.
Advancements in the history of tape drives
| Year | Manufacturer | Model | Advancements |
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| 1951 | Remington Rand | UNISERVO | First computer tape drive | | 1952 | IBM | 726 | Use of plastic tape (cellulose acetate); 7-track tape recording 6-bit bytes | | 1958 | IBM | 729 | Separate read/write heads providing transparent read-after-write verification . | | 1964 | IBM | 2400 | 9-track tape that could store every 8-bit byte plus a parity bit. | | 1970s | IBM | 3400 | Auto-loading tape reels and drives, avoiding manual tape threading; Group code recording for error recovery at 6250 bit-per-inch density | | 1972 | 3M | QIC-11 | Tape cassette (with two reels) | | 1974 | IBM | 3850 | Tape cartridge (with single reel)
First tape library with robotic access | | 1980 | Cipher | (F880?) | RAM buffer to mask start-stop delays | | 1984 | IBM | 3480 | Internal takeup reel with automatic tape takeup mechanism.
Thin-film magnetoresistive (MR) head. | | 1984 | DEC | TK50 | Linear serpentine recording | | 1986 | IBM | 3480 | Hardware data compression (IDRC algorithm) | | 1987 | Exabyte/Sony | EXB-8200 | First helical digital tape drive.
Elimination of the capstan and pinch-roller system. | | 1993 | DEC | Tx87 | Tape directory (database with first tapemark nr on each serpentine pass). | | 1995 | IBM | 3570 | Head assembly that follows pre-recorded tape servo tracks (Time Based Servoing or TBS)
Tape on unload rewound to the midpoint - halving access time (requires two-reel cassette, resulting in lesser capacity)
| | 1996 | HP | DDS3 | Partial Response Maximum Likelihood (PRML) reading method - no fixed thresholds | | 1997 | IBM | VTS | Virtual tape - disk cache that emulates tape drive | | 1999 | Exabyte | Mammoth-2 | The small cloth-covered wheel cleaning tape heads.
Inactive burnishing heads to prep the tape and deflect any debris or excess lubricant.
Section of cleaning material at the beginning of each data tape.
| | 2000 | Quantum | Super DLT | optical servo allows more precise positioning of the heads relative to the tape. | | 2003 | IBM | 3592 | Virtual backhitch | | 2003 | Sony | SAIT-1 | Single-reel cartridge for helical recording | | 2006 | StorageTek | T10000 | Multiple head assemblies and servos per drive | | 2007 | IBM | 3592 | Encryption capability integrated into the drive | | 2008 | IBM | TS1130 | GMR heads in a linear tape drive |
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