01 Apr 2015

Why LTO storage is so good for archiving video

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HP LTO-6 Ultrium HP LTO-6 Ultrium Hewlett Packard / RedShark News

Index

Phil Rhodes gives us the technical run-down on LTO storage and why you should reconsider storing your valuable footage on hard drives.

The question of what to do with original digital camera material once it's been shot has been vexing for some time. From back-bedroom shorts to blockbusters, it's easy to end up with data that represents far more work than can reasonably be risked on the reliability of a single hard disk.

Achieving reliable storage is at least as much about technique and procedure as it is about technology. Even so, the requirement for a bit bucket that's cheap enough to put on a shelf for long periods, that will remain reliable over those long periods, with high capacity and acceptably fast, seems likely to grow. Camera systems, after all, are continuously being developed to producer greater dynamic range, resolution and frame rate. Welcome as it is, that can only mean more data and, very possibly, an exponentially increasing amount of data (even just assuming linear increases in resolution).

To put this in context, the only magnetic video tape format for which new machinery can be easily found is HDCAM SR and it's already out of manufacture. Along with one or two obscure data tape formats still in use in upscale corporate IT infrastructure, LTO is among the last magnetic tape storage formats made. Tape, it seems, is not nearly as lamented a technology as film was.

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The technology itself has been fast and capable for some time; particularly, the associated standardisation process is a lovely thing to behold. The late 90s seemed, in the moment, to be a great time for data tape, with performance and capacity leaping away from the sluggish and unreliable formats of old. Compared to the cooperation and standardisation that's happening now, though, the mixed bag of physical tape formats that we used to deal with, along with the wide variety of data formatting options when actually writing files to tape, was something of a disaster.

The Hardware

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The actual LTO tape drive itself, as well as the media it uses, is an advanced expression of modern mechatronics. A lot of historic data tape formats used spinning heads, similarly to a video recorder. The idea here is to increase the speed with which the head and tape move past each other, without having to move the tape very rapidly. The higher the tape speed, broadly speaking, the higher the maximum frequency of information that can be recorded on it and, therefore, the greater the data rate. This works OK, but as DAT users might recall, spinning heads are power hungry, costly, make it far harder to calibrate mechanisms (such that everything will read and write tapes compatibly) and present the possibility of some really serious tape chewing if things go wrong.

Instead, the L in LTO stands for linear; the heads are static, as in audio tape recorders of old, and the tape moves past them. The analogue audiotape metaphor falls apart quickly after this, though, because LTO tape drives have not two (as for stereo) but sixteen heads, each of which writes a portion of a track each time the tape moves past, then moving to cover a different portion of the track. As such, the tape moves back and forth along its length several times in order to write its entire capacity, meaning the tape can move quickly, maximising bandwidth and, with both that and the sixteen heads working in concert, there is no need for tricky spinning heads.

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One particularly key aspect of linear tape formats is that it is possible to include what would, on an old audio tape format, be called a confidence replay head. LTO drives read the data they've just written to confirm that it's been written correctly, which isn't something that's really possible with a helical-scan tape format. This adds a lot to the reliability of the LTO format. There are arguments that this feature, which will detect hard write errors, doesn't completely obviate software verification, which can also detect problems in shipping the data to the drive, but it's another welcome feature nonetheless.

The sixth and current generation of LTO offers 2500GB of native capacity, which we need to be cautious about because many statistics for LTO quote a capacity using the system's inbuilt compression and using base-10 numbers. Headline capacities are often given, assuming the compression is switched on and working, which is why many LTO6 tapes are marked with a capacity of more than six terabytes. The real-world formatted capacity of an LTO6 cartridge without compression is actually about 2.29 terabytes. The compression won't work, or at least won't work well, with video data that's already compressed. The compression can be disabled at the point of formatting a tape, although the drives should automatically disable it for data that can't be compressed. LTO6 writes this data at a theoretical maximum of 160 megabytes per second, although real-world rates, given the usual small inefficiencies, are about 140 on large files.



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Phil Rhodes

Phil Rhodes is a Cinematographer, Technologist, Writer and above all Communicator. Never afraid to speak his mind, and always worth listening to, he's a frequent contributor to RedShark.

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