Tape Technologies (LTO, DLT & AIT)
There are three technologies we should consider when looking at physical tape archiving: LTO, DLT, and AIT.
LTO: Linear Tape Open (LTO) is a standard jointly developed by HP, IBM, and Seagate and because the standard is "open" there can be multiple sources of media and drives enabling compatibility from different vendors' products.
Although, how well this "open standard" works in practice is a matter for some debate. While no one seems to disagree with the compatibility of the -media", the standard is not quite so rigorous for the drive itself, and a tape written by one vendor's product may not be read correctly by another vendor's product.
There are three incarnations of the LTO technology which reside on a relatively compact and robust tape cartridge.
- LTO 1 using 2:1 compression has a compressed capacity of 200GB and a data transfer rate of 15 to 30MB/sec.
- LTO 2 again with 2:1 compression has a maximum capacity of 400GB and a transfer rate of 30 to 60MB/sec.
- LTO 3 also doubles the capacity of the previous generation with a maximum 2:1 compression capacity of 800GB and 68 to 136MB/sec transfer rate which again is dependent on the compression ratio.
There is a fourth generation of the LTO technology on the drawing boards with twice the capacity and transfer speed of LTO 3.
LTO 3 cartridge physical specifications:
- 105 x 102 x 21mm single reel.
- Four bands of 96 tracks for a total of 384 tracks -- each of the bands are bounded by servo information.
- Dual servo information, primary and secondary, written at the factory.
- Longitudinal servo information is written within the servo frames for precise locations down the length of the tape.
- Two levels of error correction can enable data to be recovered from longitudinal media scratches for example and real time verification of data is supported with a Read-While-Write capability.
- 4KB of integrated non-volatile memory containing information for calibration, initialisation and manufacturers data that is read by passive non contact RF.
DLT: Digital Linear Tape (DLT) is a standard that has evolved quite dramatically and has quite a large proportion of the tape market. The capacity of the original DLT tapes are quite passé nowadays but the new SuperDLT tapes have increased in capacity pretty much in sync with LTO. DLT and Super DLT (SDLT) are not open standards but are tightly controlled by Quantum so one would arguably expect better compatibility between different vendors' drives. The current DLT series of tape backup drives are considered the "value" series and currently do not have a high enough capacity or transfer rate to satisfy many large corporate requirements with a peak capacity of 160GB compressed and a maximum sustained transfer rate of 16MB/sec.
- SDLT 320 at 2:1 compression has a capacity of 320GB and a sustained transfer rate of 16 to 32MB/sec.
- SDLT 600 has a native capacity of 300GB or 600GB compressed with a sustained transfer rate of 36 to 72MB/sec depending on compression.
The DLT camp has a very ambitious roadmap -- they don't just give us a peek at the next product but manage, using their crystal ball, to let us peek four generations ahead. Following SDLT 600 are DLT-S4, DLT-S5, DLT-S6 and DLT-S7 ranging from 1.6TB of compressed storage and 120MB/sec transfer rate in the case of the S4 up to a whopping 12 to 14TB and 800 to 1000MB/sec transfer rates.
The physical specifications for the SDLT 600 are:
- 104.1 x 104.1 x 25.4mm
- Recording format is 640 tracks in a serial serpentine configuration.
- Tape length is 630m
AIT: Advanced Intelligent Tape (AIT) is Sony's foray into the tape backup arena and while some may argue that previous generations capacity and transfer rates are a little puny when compared to SDLT and LTO3, Sony has provided an answer in the form of AIT-4 and SAIT. Like the DLT camp Sony has stuck an S for Super in front of the previous generations name and boosted capacity and performance markedly.
- AIT-1 relied on greater compression than the competitors to realise good capacity with a native capacity of 35GB and a compressed capacity of 90GB -- transfer speeds were four and 10.4MB/sec respectively. There was a "turbo" version of AIT-1 with 104GB compressed and a 50 percent faster transfer rate.
- AIT-2 had a native/compressed capacity of 50/130GB and transfer rates of 6/15.6MB/sec.
- AIT-3 had a native/compressed capacity of 100/260GB and transfer rates of 12/30MB/sec.
- AIT-4 is the latest generation and has a native/compressed capacity of 200/520GB with a native transfer rate of 24MB/s. Sony is quite ambitious with its product roadmaps, and by 2010 it says its SAIT-drive will have a native capacity of 4TB, and by 2008 AIT-6 will hit 800GB native.
- SAIT-1 the first generation of SAIT, is certainly impressive with a native capacity of 500GB or 1.3TB compressed with transfer rates of 30 and 78MB/sec respectively.
