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Reliability questions plague solid state

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Just how reliable are the NAND flash cells in solid-state drives (SSDs)? One of flash memory's drawbacks has been that it permits only a finite number of writes before it becomes unusable.

SSD reliability came under attack earlier this year when Avi Cohen, head of research and managing partner at Boston-based Avian Securities LLC, leveled the charge in a report that Dell laptops with SSDs were suffering from failure rates of between 10% and 20% and that the return rate on these laptops was between 20% and 30%.

Mark Farley at Dell EqualLogic responded in a blog that Dell's SSDs "are showing the kind of reliability that everyone expected of storage with no moving parts. They are just as reliable as rotating disk drives, if not more so."

Farley pointed out that Dell's SSDs don't use low-cost, consumer multilayer flash but single-level flash, which has better reliability. So these drives don't have disastrous end-of-life failures from wear-out. In fact, he says, Dell's next-generation SSD products will have performance levels that could exceed those of 7,200 rpm disk drives.

Flash drives do wear out, acknowledges David Flynn, CTO at Fusion-io. The startup's ioDrive offers a solid-state I/O NAND clustering technology that has been adopted by Hewlett-Packard (HP) for its enterprise-class BladeSystem c-Class system.

"Disk drives wear out, too, but people don't

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like to talk about that," says Flynn. "The thing about disk drives is you don't know when they'll wear out. Their average usable life span is five years. But the wear-out on flash drives is predictable, and we believe that predictable wear-out is much better than unpredictable [wear-out]."

Fusion-io's Flynn also notes that wear-out isn't a catastrophic failure. "Wear-out is bits getting stuck," he says, "and the problem of stuck bits can be easily corrected using ECC code."

Of course, wear-out is also related to where SSDs are in the storage infrastructure. Patrick Eitenbichler, director of marketing at HP's StorageWorks Division, says while SSD has value as a Tier 0 for I/O-intensive applications, "as Tier 1 storage, it wears out very quickly."

This was first published in August 2008

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