SSDs Take Over The World, Bit by Bit

This year’s Flash Memory Summit has just ended and I’ll be devoting several blog posts to it because of all the great information I collected in a mere three days. A truly worthwhile event. Consider attending next year.

Last year, the Flash Memory Summit was all about chips. This year, it was pretty much all about NAND-Flash-based SSDs (solid-state disks)—assemblages of semiconductors that emulate rotating, mechanical hard disk drives (HDDs) at the interface level so that they can plug into existing interfaces in most systems and can be operated at some level by most operating systems. SSDs do not directly replace HDDs because they cost more per bit (greater $/Gbyte) than do HDDs. However SSDs deliver substantial performance benefits.

Consumer-level products such as PCs and PVRs (personal video recorders) may not benefit much from faster disk drives. Enterprise systems do. Enterprise computing clients know precisely what a second’s worth of delay costs in their business (sometimes a microsecond’s delay costs big money). It’s part of an IT department’s job to justify costs versus performance for the gear it buys, so the value of time already goes into the purchase equation.

That’s why so many 15,000-rpm HDDs are going into enterprise systems. That’s why drive-array vendors “short-stroke” HDDs, to pare down average access time at the expense of capacity. One IBM drive array with 800 Tbytes (that’s terabytes, folks) mixes fast-access short-stroke drives and full-stroke drives for a total of 10,000 spindles: to cut access times and boost data bandwidth.

Moore’s Law isn’t helping hard disk speeds at all. And even though HDD capacity has grown faster than Moore’s Law, thanks to some true technical wizardry from the HDD industry, HDD performance (as measured in IO transactions per second or IOPS) has hardly improved at all. Hence the need to trade off HDD capacity for some additional performance, albeit minimal.

So while SSDs cannot compete on $/Gbyte, they can indeed compete on performance. Professor Sang-Won Lee of Sungkyunkwan University presented some analysis of SSD versus HDD performance. With his performance numbers for online transaction processing, he claimed that one SSD could outperform eight 15,000-rpm HDDs today and will be able to outperform 20 to 40 such HDDs in a year or two. Note that the storage capacity will definitely be less for the SSD as the ten HDDs, but remember that vendors are short-stroking HDDs to get more performance at the expense of capacity. Performance has value in this market.

SSDs also outperform HDDs when it comes to power consumption. One SSD consumes about 5W. As stated in the previous paragraph, equivalent HDD performance requires at least eight HDDs, which together consume 104W. So the SDD delivers the same number of IOPS with only 5% of the power consumption—a green solution, at least for the enterprise.