Taking lots of cheap drives for a spin

Much of the debate about power conservation in computing centres on servers. Being big, power-hungry machines, they burn through lots of energy – which is why you need heat sinks, fans and so on to cool them down. They also represent a big potential energy saving, because virtualising operating systems can potentially claw back 70 to 80 per cent of the power that a server is wasting by not using all of its processing power.

But now some experts are talking about another IT resource that tends to use lots of energy without doing very much: storage.

Imagine that you are in an archiving environment, where lots of data must be stored for posterity without being accessed very often. Traditionally, you would use tape storage, but that has several drawbacks.

First, tapes cannot shift data around very quickly. In an age of multimedia storage, that is bad because when you want to access some low-latency audio or video footage, lots of data has to move around very quickly.

Second, unless you are archiving to more than one tape at a time, you are relying on a single point of failure, whereas Raid disk storage can provide redundancy relatively easily.

And third, it can be difficult to perform complex searches across tape storage. "The largest growing archive is unstructured data," said Chris Santilli, chief technology officer of Copan Systems. "But try to do a search on that tape jukebox and get all of the records."

Santilli has an axe to grind. His company sells virtual tape library systems – huge banks of relatively inexpensive disk drive storage arrays designed to substitute tapes for disks. That gives you the benefit of fast random access, and redundancy, but at what cost?

Tapes do not use any power when they are not being used. But disks, on the other hand, spin. And spinning lots of disks at once can take up a lot of power. Storage analyst Hamish MacArthur, of MacArthur Stroud International, said that tier one disk storage (consisting of arrays of high-speed disks connected via fibre channel links) delivers around 4.1 Terabytes for every kilowatt of power it consumes. Tier two storage (which uses less expensive but slower Serial ATA drives) delivers 17.4 Tb per Kw.

However, Copan's systems deliver 91.4 Tb for a Kw of power. That is because the technology stops spinning the disks that are not being used, only spinning them when their data is requested or when they are being tested for data integrity.

"We have heard IBM talking about this too and HDS [Hitachi Data Systems] has a solution here," said MacArthur. "When you look at it from a virtual tape perspective, one thing that more managers want to do is to put more images onto disk before putting it onto tape. That is great, but why have it spinning all the time?"

This will not be a solution for everyone – Copan designates a minimum storage capacity of 20Tb before its systems become interesting to customers – but then the power considerations about storage will be more pronounced as storage capacity goes up anyway. For those large storage users that want an alternative to tape (or at least another tier of storage in between), large numbers of idle drives could be worth a look.