Tape drive

Tape drive
DDS tape drive. Above, from left right: DDS-4 tape (20 GB), 112m Data8 tape (2.5 GB), QIC DC-6250 tape (250 MB), and a 3.5" floppy disk (1.44 MB)

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 sequential access storage, unlike a disk drive, which provides random access storage. A disk drive read/write head can move to any position on the disk in a few milliseconds, but a tape drive must physically wind tape between reels to read any one particular piece of data. As a result, tape drives have very slow average seek times. For sequential access once the tape is positioned, however, tape drives can stream data very fast. For example, as of 2010 Linear Tape-Open (LTO) supported continuous data transfer rates of up to 140 MB/s, comparable to hard disk drives.

Contents

Design

An external QIC tape drive.

Tape drives can range in capacity from a few megabytes to hundreds of gigabytes of uncompressed data.

As some data can be compressed to a smaller size than the files on hard disc, it has become commonplace when marketing tape drives to state the capacity with the assumption of a 2:1 compression ratio; thus a tape with a capacity of 80 GB would be sold as "80/160". The true storage capacity is also known as the native capacity or the raw capacity. IBM and Sony have also used higher compression ratios in their marketing materials. The compression ratio actually achievable depends on the data being compressed. Some data has little redundancy; large video files, for example, already use compression technology and cannot be compressed further. A sparse database, on the other hand, may allow compression ratios better than 10:1.

Tape drives can be connected to a computer with SCSI (most common), Fibre Channel, SATA, USB, FireWire, FICON, or other[1] interfaces. Tape drives are used with autoloaders and tape libraries which automatically load, unload, and store multiple tapes, increasing the volume of data which can be stored without manual intervention.

Some older tape drives were designed as inexpensive alternatives to very expensive 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 linear serpentine layout for writing data to tape, and was made unnecessary by decreasing disk drive prices anyway.

Reliability

Gartner Group estimated that 10 to 50 percent of all tape restores fail. Storage Magazine and Gartner reported that 34% of surveyed companies never test a restore from tape, and of those that do test, 77% experienced tape backup failures.[2]

Performance problems

A disadvantageous effect of "shoe-shining" occurs during read/write if the data transfer rate falls below the minimum threshold at which the tape drive heads were designed to transfer data to or from a continuously running tape. In this situation, the modern fast-running tape drive is unable to stop the tape instantly. Instead, the drive must decelerate and stop the tape, rewind it a short distance, restart it, position back to the point at which streaming stopped and then resume the operation. If the condition repeats, the resulting back-and-forth tape motion resembles that of shining shoes with a cloth. Shoe-shining decreases the attainable data transfer rate, and drive and tape life.

In early tape drives, non-continuous data transfer was normal and unavoidable - computer processing power and memory available were usually insufficient to provide a constant stream. So tape drives were typically designed for so called start-stop operation. Early drives used very large spools, which necessarily had high inertia and did not start and stop moving easily. To provide high start, stop, and 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 started, stopped and 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 drives of the 1980s introduced the use of an internal data buffer to somewhat reduce start-stop situations. These drives are often referred to as tape streamers. The tape was stopped only when the buffer contained no data to be written, or when it was full of data during reading. As faster tape drives became available, despite being buffered the drives started to suffer from the shoe-shining sequence of stop, rewind, start.

Most recently, drives no longer operate at a single fixed linear speed, but have several speeds. Internally, they implement algorithms that dynamically match the tape speed level to the computer's data rate. Example speed levels could be 50 percent, 75 percent and 100 percent of full speed. A computer that streams data slower than the lowest speed level (e.g. at 49 percent) will still cause shoe-shining.

Media

Magnetic tape is commonly housed in a casing known as a cassette or cartridge—for example, the 4-track cartridge and the compact cassette. The cassette contains magnetic tape to provide different audio content using the same player. The outer shell, made of plastic, sometimes with metal plates and parts, permits ease of handling of the fragile tape, making it far more convenient and robust than having spools of exposed tape. Simple Compact Cassette audio tape recorders were commonly used for data storage and distribution on home computers at a time when floppy disk drives were very expensive. The Commodore Datassette was a dedicated data version of the same system.

History

Year Manufacturer Model Capacity Advancements
1951 Remington Rand UNISERVO 224 kB 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.[3] As of January 2009, the Computer History Museum in Mountain View, California has working IBM 729 tape drives attached to its working IBM 1401 system.[4]
1964 IBM 2400 9-track tape that could store every 8-bit byte plus a parity bit.
1970's 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 20 MB Quarter-inch cartridge tape cassette (with two reels)
1974 IBM 3850 Tape cartridge (with single reel)

First tape library with robotic access [5]

1978 Commodore International Commodore Datasette 1978 kB Use of standard audio cassettes
1980 Cipher (F880?) RAM buffer to mask start-stop delays[6][7]
1984 IBM 3480 Internal takeup reel with automatic tape takeup mechanism.

Thin-film magnetoresistive (MR) head. [5]

1984 DEC TK50 94 MB Digital Linear Tape (DLT) [8]
1986 IBM 3480 Hardware data compression (IDRC algorithm) [5]
1987 Exabyte/Sony EXB-8200 2.4 GB 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). [9]
1995 IBM 3570 Head assembly that follows pre-recorded tape servo tracks (Time Based Servoing or TBS) [10]

Tape on unload rewound to the midpoint — halving access time (requires two-reel cassette, resulting in lesser capacity) [11]

1996 HP DDS3 12 GB Partial Response Maximum Likelihood (PRML) reading method — no fixed thresholds[12]
1997 IBM VTS Virtual tape — disk cache that emulates tape drive [5]
1999 Exabyte Mammoth-2 60 GB 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 110 GB optical servo allows more precise positioning of the heads relative to the tape.[13]
2003 IBM 3592 Virtual backhitch
2003 Sony SAIT-1 500 GB Single-reel cartridge for helical recording
2006 StorageTek T10000 Multiple head assemblies and servos per drive [14]
2006 IBM 3592 Encryption capability integrated into the drive
2008 IBM TS1130 GMR heads in a linear tape drive
2010 IBM TS2250 LTO Gen5 Linear Tape File System (LTFS), which allows accessing files on tape in the same way as on a disk filesystem
2011 Oracle T10000C 500 - 5000 GB  ?[15]

Market Trends

In 2007, Gartner analyst Dave Russell predicted that recovery will move from tape to online disk-based storage by 2011, causing a major shift in the backup market.[16]

Future Capacity

Tape drives have yet to reach their maximum capacity.

In 2011, Fujifilm and IBM announced that they had managed to record 29.5 billion bits per square inch with magnetic tape media developed using the BaFe particles and nanotechnologies allowing for an uncompressed tape drive of 35TB.[17][18] The technology is not expected to be commercially available for at least another decade.

Notes

  1. ^ Historical interfaces include also ESCON, parallel port, IDE, Pertec.
  2. ^ Tape: A Collapsing Star, by Randy Chalfant, March 18, 2010. MainframeZone.
  3. ^ http://www.research.ibm.com/journal/rd/255/ibmrd2505ZD.pdf
  4. ^ http://ed-thelen.org/1401Project/1401RestorationPage.html
  5. ^ a b c d http://www-03.ibm.com/ibm/history/exhibits/storage/storage_fifty.html
  6. ^ http://www.freepatentsonline.com/4500965.html
  7. ^ http://www.aceware.iinet.net.au/acms/Images/BooksHiRes/000732.jpg
  8. ^ http://www.xenya.si/sup/info/digital/MDS/jun99/Cd2/DECSTA/422AAMG1.PDF
  9. ^ http://alumnus.caltech.edu/~rdv/comp-arch-storage/FAQ-1.5.html
  10. ^ http://www.research.ibm.com/journal/rd/474/biskeborn.html
  11. ^ http://maben.homeip.net:8217/static/computers/backup/tsm/links/TSM%20quickfacts.txt
  12. ^ http://www.freepatentsonline.com/6970522.html
  13. ^ http://findarticles.com/p/articles/mi_m0BRZ/is_12_19/ai_58926467/pg_2
  14. ^ http://www.thic.org/pdf/July06/sun.rraymond060718.pdf
  15. ^ oracle.com - Storagetek T10000 family, tape cartridge, key benefits
  16. ^ Recovery will move to disk-based, manager of managers approach by 2011. Dave Russel, Gartner Group. 2007
  17. ^ "FUJIFILM BARIUM-FERRITE MAGNETIC TAPE ESTABLISHES WORLD RECORD IN DATA DENSITY: 29.5 BILLION BITS PER SQUARE INCH". Fujifilm. January 22, 2010. http://www.fujifilmusa.com/press/news/display_news?newsID=879807. Retrieved 13 July 2011. 
  18. ^ Harris, Robin (January 24, 2010). "A 70 TB tape cartridge: too much, too late?". ZDNet. http://www.zdnet.com/blog/storage/a-70-tb-tape-cartridge-too-much-too-late/769. Retrieved 13 July 2011. 

References


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