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DLT Digital Linear Tape Technology


DLT Drive and Media

DLT derived from the TK50 and TK70 1/2" tape cartridges that DEC (Digital Equipment Corporation) had developed by the late Fred Hertrick for its MicroVax II workstations in the 1980's. DEC's tape manufacturing and development was taken over by Quantum Corporation in 1994. Since the initial release of TK50/TZ30 back in 1984 the capacity of the media and format has grown from 100MB per tape native (un-compressed) to the DLT-S4 native capacity of 800GB per tape.

Development of DLT split a few years back with the development of Super DLT (S-DLT) which targeted the high end server storage market, and DLT1 (later VS80, VS160...) which were a lower manufacturing cost drive aimed at a gap left by a slow down in the progress of 4mm DAT in the 40/80GB storage range and beyond.

DLT1 was originally produced by a breakaway company from Quantum named Benchmark, though Quantum soon bought the company back into the fold and progressed with development of its product range.

The most recent addition to the DLT range is the DLT-S4, with a compressed storage capacity of 1.6TB. Quantum have stated that either this or DLT-S5 will be the end of the line for DLT with LTO Ultrium having won the battle for dominance in this market.

How DLT Works

DLT uses linear/serpentine recording method (as opposed to the helical scan methodology employed by DAT, AIT, Exabyte). The recording media is 1/2" (1.27cm) and housed in a cartridge approximately 4" (10.5cm) square. The recording capacity of the media has increased immensely since it's inception from 600MB on CompacTape to 800GB on DLT-S4.


Above: Linear/Serpentine recording


Above: DLT tape path in drive mechanism

As with many other media types where linear recording is used, data is recorded as a sequence of tracks that run alternately forwards and backwards along the tape. This technique is known as serpentine recording. Unlike with most formats of helical recording drives, by the time the logical end of tape is reached, the media is physically back at the start.

The media is held on a single reel, rather than in a cassette, and the tape is drawn from within the cartridge case and wound around a take-up reel within the drive.

DLT Data Recovery

DLT has long been accepted as a tough and robust medium, and its pedigree in the middle to high end storage market proves this. As with any storage medium, however, things can go wrong. Drive failure can damage a tape, over use can result in media failure, dirt and dust can have a damaging abrasive effect, and human error (forgetting to write protect a tape, writing to the wrong tape etc.) is an ever present factor.

Data recovery from physically damaged tapes is complicated by the DLT using the serpentine manner of recording. Since multiple tracks are recorded along the tapes, alternately backwards and forwards, damage to the tape has the effect of causing read problems in multiple positions. Quite simply, if there is 1GB of data recording along each track of a tape and the media is damaged 50% of the way along its length then there will be problems reading the tape at 500MB, 1.5GB, 2.5GB and so on.

This is most noticeable when a tapes is snapped or stretched, but if there are problems resulting from over use then any point at which the recordable medium becomes worn will probably traverse several tracks and give a similar effect.

Where a DLT is re-initialised or over-recorded there can be the effect that many gigabytes of data are lost as the result of a few megabytes (or even a few bytes) being written. As with all modern tape drives the last thing you write is the last thing you can read. If you re-initialise a tape then all older data is deemed to be unavailable and can only be accessed by using a data recovery service.

DLT Drive Type and Capacities

DriveNative CapacityData Rate (MB/s)Date IntroducedMedia Type
TK50/TZ30 100MB 0.045 1984 CompacTape
TK70 300Mb 0.045 1987 CompacTape II
DLT260/TK85 2.6GB 0.8 1989 DLT III (mid-brown)
DLT600/TK86 6GB 0.8 1991 DLT III
DLT2000 10GB 1.25 1993 DLT III
DLT2000XT 15GB 1.25 1995 DLT IIIXT (white)
DLT4000 20GB 1.5 1994 DLT IV (dark brown)
DLT7000 35GB 5 1996 DLT IV
DLT8000 40GB 6 1999 DLT IV
SDLT 220 110GB 10 2000 SDLT I (green)
SDLT 320 160GB 16 2002 SDLT I
SDLT 600 300GB 36 2004 SDLT II
DLT-S4 800GB 60 2006 S4
Benchmark / Value Range
DLT1 40GB 3 1999 DLT IV
DLT-VS80 40GB 3 2001 DLT IV
DLT-VS160 80GB 8 2003 VS1
DLT-V4 160GB 10 2005 VS1

Last Updated (Wednesday, 14 October 2015 15:55)

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