Monday, 28 July 2008

Claimed 200 year media life

Graeme Pow spotted the announcement a few weeks back of Delkin's Archival Blu-ray media:
"New from Delkin Devices is Archival Gold Blu-ray, a recordable disc that guarantees to preserve data safely for over 200 years. In addition to unprecedented longevity standards, Delkin BD-R boast a market-leading read/write speed of 4x, enabling a 25GB burn to be completed in only 23 minutes -- and the disc is coated in ScratchArmour, a scratch-proof coating that claims to be 50 times better than other coatings. "
Italicised part is a direct quote from Delkin's site. Delkin also claim at the site linked above:
"Delkin archival Blu-ray (BD-R) discs offer the longest guaranteed protection over time."
There is a fair amount of scepticism about such claims, for example see the cdfreaks forum thread on archival gold DVD-Rs...

So what does this mean?
  • Well firstly they clearly haven't run the media for 200 years, so presumably this is the result of some accelerated aging tests extrapolated into "normal" use.
  • Secondly, if the disk did fail in however many years into the future, who would your successors claim from, and what? I don't have a copy of the "guarantee", but my guess is you'd at best get replacement media cost, not the lost content value (a wag on cdfreaks suggested that the guarantee was only available to the original purchaser ;-).
  • Thirdly, if your successors still have the disk in 200 years, what are the odds of having hardware and software to read it? Close to zero I guess.
  • Fourthly, would you even know what's on it? Oh, you wrote a label on the outside, did you? Or perhaps wrote in marker on the disk? Or...
It might seem like a good thing to have disks that last for much longer than the devices. After all, that would remove one error source, and would mean you could use your devices to help in the transfer to newer media. However, assuming the media really were robust, this might lead to a temptation to ignore the transfer, until you (or your descendants or successors) suddenly find your last Blu-ray device has failed!

In practice, of course, relying on any such claims would be foolish. It's not a bad idea to use good quality media, but I would want to choose it based on testing from an independent lab rather than manufacturers' claims. And then it should fit into a well-planned strategy of media management. I guess it would be wise to keep some archival media of the same type but non-critical contents, and test them from time-to-time, watching for increased error rates.

Interestingly, I've just re-read (yet again) the OAIS section 5, which covers these issues. I can't say it's a lot of help for most people wanting advice. It refers to the whole issue as one of "Digital Migration" (not the customary use of the word migration in digital preservation circles these days, where it is mostly used in contrast to emulation), and lists 4 types:
  • Refreshment: A Digital Migration where a media instance, holding one or more AIPs or parts of AIPs, is replaced by a media instance of the same type by copying the bits on the medium used to hold AIPs and to manage and access the medium. As a result, the existing Archival Storage mapping infrastructure, without alteration, is able to continue to locate and access the AIP.
  • –Replication: A Digital Migration where there is no change to the Packaging Information, the Content Information and the PDI. The bits used to convey these information objects are preserved in the transfer to the same or new media-type instance. Note that Refreshment is also a Replication, but Replication may require changes to the Archival Storage mapping infrastructure.
  • –Repackaging: A Digital Migration where there is some change in the bits of the Packaging Information.
  • Transformation: A Digital Migration where there is some change in the Content Information or PDI bits while attempting to preserve the full information content.
So that's clear, then. I think the first two are merely replacement by identical media. Later text makes clear that the third is the case we would be considering here, ie the replacement of one media type with another, while the fourth would represent what we would currently call migration, ie making some change to the information object in order to preserve its information content. However, despite quite a bit of discussion on scenarios of the various digital migration types, I could not see much in the way of good practice advice there.


Post a Comment

Please note that this blog has a Creative Commons Attribution licence, and that by posting a comment you agree to your comment being published under this licence. You must be registered to comment, but I'm turning off moderation as an experiment.