Facebook Experimenting With Blu-ray As a Storage Medium

s122604 links to CNN's explanation of what may be the future of cold (or at least lukewarm) storage at Facebook, which is experimenting with massive arrays of Blu-Ray discs for seldom-accessed user files. Says the report: The discs are held in groups of 12 in locked cartridges and are extracted by a robotic arm whenever they're needed. One rack contains 10,000 discs, and is capable of storing a petabyte of data, or one million gigabytes. Blu-ray discs offer a number of advantages versus hard drives. For one thing, the discs are more resilient: they're water- and dust-resistant, and better able to withstand temperature swings. Their data can be restored more quickly, and they're easier to transport. Most important, though, is cost. Because the Blu-ray system doesn't need to be powered when the discs aren't in use, it uses 80% less power than the hard-drive arrangement, cutting overall costs in half.

Comments

Why not just use hard drives and then store...

[Anonymous Coward]

... those drives offline or come up with a system to power up the drives via custom san hardware when you want to access them? With facebooks cash it should be do-able.

Re:Why not just use hard drives and then store...

[lucm]

When you deal with cold storage you have to look at things from a node level, not in global storage size. If your basic unit is a 50GB device instead of a 4TB device, this means that each request you make to recall data has a much smaller footprint.

Let's say that each stored account takes up 1GB of space. That's 50 accounts per BD drive, and 4000 accounts per hard disk. This means that when some dude comes out of jail and tries to access the photo his mom posted on his Facebook wall in 2010, there are 3999 accounts that are pulled out of their coma with it for no reason. On a BD that's only 49.

As long as you partition stuff properly it's unlikely that a single request will span multiple BD drives. You may have to deal with clusters of BD disks and this requires a bit of tuning, but even with the best indexing system in the world you can't power up only part of a hard disk. So BD is a clear winner here, especially if to that footprint issue you add the fact that spinners die quickly when you keep playing with the on/off switch.

Bytes are bytes when you live in a software world. But physical factors and limitations come into play when you deal with storage, and that's why most people with a software background can see WTF where there is instead good engineering.

Re:Why not the "boring" Tape storage?

[DoomSprinkles]

That's not really how tape systems work. Generally they keep an index online so you can tell the tape system to pop in a specific tape and goto a specific position, longest load times... in real world that i've personally witnessed... 10 mins

Well, that does it for Facebook.

[jd]

Not that I had any trust in them anyway.

Blu-Ray, and indeed any modern optical storage, is very short-lived precisely because it's designed to be cheap. The laser disks used to store the Doomsday Project in Britain were still readable after 20 years. Modern optical storage decays typically within 5. Less, as the density goes up. And failures take out far larger percentages of the storage.

Magnetic tape is still the only trusted long-term backup medium. I wouldn't suggest it for something like Facebook purely because of seek times, but it's hard to think of any viable alternative.

With Blu-Ray, to guarantee to avoid complete disk loss, you'd have to be re-archiving the entire archive annually. That adds an enormous invisible cost to the project. They're not going to do that. Which means there's guaranteed loss of backups. How much depends on the exact storage conditions but it won't be pretty.

As for better ability to withstand conditions, it again comes down to the nature of the storage. Optical disks are highly vulnerable to a lot of things that hard drives are not. Overall, optical storage usually performs very badly in comparison, as the things hard drives are vulnerable to are cheaply avoided but the things optical storage can be attacked by are usually a lot harder to deal with.

I'm sure you're aware that none of the above formats (tape included) are considered "archival quality" - they just don't have the sort of durability required by that categorization. No known digital format does and there's nothing you can do to stabilize them. It's a big research area. For now, tape is considered the only method that is economic and durable, with the lowest loss of data per failure.