No sooner have we got used to the idea of SCSI hard drives being replaced in enterprise arrays by serial versions of the once PC-focused ATA spec, than it's time to change back to SCSI - though once again, in a serial-attached (SAS) flavour.

So says Mark Ward, the CEO of storage array developer Copan Systems. Drives will also get physically smaller, he adds.

"The balance [in enterprise arrays] has gone from 30 percent SATA to 70 percent in three years," he says, "But looking forward, the industry will probably make SAS a replacement for SATA, as the vendors believe that SAS will eventually be cheaper to manufacture.

"And 2.5 inch will replace 3.5 inch - at the moment 3.5 is still around twice as efficient, but they will cross over. We're also looking at solid-state - in three to five years it could be cheaper than hard disk."

Copan's speciality is low-power long-term online storage. It was one of the pioneers of MAID (massive arrays of idle disk), which keeps data online but turns off hard drives to save power when they're not being accessed. (If a drive's not been accessed in 30 days, it is powered up for a check.)

MAID to WORI?

This kind of storage is ideal for "persistent data - that's write once, read infrequently," says Ward. It's data which you won't access very often, but when you do need to get at it, you'll want to get to it easily and reasonably quickly. In the past, this has been pushed off to secondary tiers of storage, often in an optical disc or tape library.

"Tape was invented to address the problems of 30 years ago. Those problems have been superseded now," says Ward. "People tried tiers, but now they're all tiered out - all they're doing is adding complexity.

"Yet everyone agrees there's transactional data and there's persistent data. For example, a major US bank lets you view your cheque images online - so it needs storage like this.

"We have large customers in social networking too - there is incredible data availability demand, and that's persistent data. Also there's things like satellite imaging where you might need to pull up an image in a hurry - we have 7.5PB at the CIA for that."

Other examples of applications that involve persistent data include e-discovery and compliance, and persistent data might still need replication for disaster recovery and de-duplication to save space, Ward adds. But he says that while there's different ways of addressing it, the basic challenge is the same for everyone.