Blade Watch

http://perspectives.mvdirona.com/2009/04/12/WhereSSDsDontMakeSenseInServerApplications.aspx

All new technologies go through an early phase when everyone initially is completely convinced the technology can’t work. Then for those that actually do solve interesting problems, they get adopted in some workloads and head into the next phase. In the next phase, people see the technology actually works well for some workloads and they generalize this outcome to a wider class of workloads. They get convinced the new technology is the solution for all problems. Solid State Disks (SSDs) are now clearly in this next phase.

Well intentioned people are arguing emphatically that SSDs are great because they are “fast”. For the most part, SSDs actually are faster than disks both in random reads, random writes and sequential I/O. I say “for the most part” since some SSDs have been incredibly bad at random writes. I’ve seen sequential write rates as low as ¼ that of magnetic HDDs but Gen2 SSD devices are now far better. Good devices are now delivering faster than HDD results across random read, write, and sequential I/O. It’s no longer the case that SSDs are “only good for read intensive workloads”.

So, the argument that SSDs are fast is now largely true but “fast” really is a misleading measure. Performance without cost has no value. What we need to look at is performance per unit cost. For example, SSD sequential access performance is slightly better than most HDDs but the cost MB/s is considerably higher. It’s cheaper to obtain sequential bandwidth from multiple disks than from a single SSD. We have to look at performance per unit cost rather than just performance. When you hear a reference to performance as a one dimensional metric, you’re not getting a useful engineering data point.

Solid State Drives remain an ideal solution for many specific uses, but as with any technology, you need to test it and see how it would fit within your infrastructure your business.

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