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Month: July 2008

I’ve written recently about the hybrid storage pool (HSP), using ZFS to augment the conventional storage stack with flash memory. The resulting system improve performance, cost, density, capacity, power dissipation — pretty much evey axis of importance.

An important component of the HSP is something called the second level adaptive replacement cache (L2ARC). This allows ZFS to use flash as a caching tier that falls between RAM and disk in the storage hierarchy, and permits huge working sets to be serviced with latencies under 100us. My colleague, Brendan Gregg, implemented the L2ARC, and has written a great summary of how the L2ARC works and some concrete results. Using the L2ARC, Brendan was able to achieve a 730% performance improvement over 7200RPM drives. Compare that with 15K RPM drives which will improve performance by at most 100-200%, while costing more, using more power, and delivering less total capacity than Brendan’s configuration. Score one for the hybrid storage pool!

As I mentioned in my previous post, I wrote an article about the hybrid storage pool (HSP); that article appears in the recently released July issue of Communications of the ACM. You can find it here. In the article, I talk about a novel way of augmenting the traditional storage stack with flash memory as a new level in the hierarchy between DRAM and disk, as well as the ways in which we’ve adapted ZFS and optimized it for use with flash.

So what’s the impact of the HSP? Very simply, the article demonstrates that, considering the axes of cost, throughput, capacity, IOPS and power-efficiency, HSPs can match and exceed what’s possible with either drives or flash alone. Further, an HSP can be built or modified to address specific goals independently. For example, it’s common to use 15K RPM drives to get high IOPS; unfortunately, they’re expensive, power-hungry, and offer only a modest improvement. It’s possible to build an HSP that can match the necessary IOPS count at a much lower cost both in terms of the initial investment and the power and cooling costs. As another example, people are starting to consider all-flash solutions to get very high IOPS with low power consumption. Using flash as primary storage means that some capacity will be lost to redundancy. An HSP can provide the same IOPS, but use conventional disks to provide redundancy yielding a significantly lower cost.

My hope — perhaps risibly naive — is that HSPs will mean the eventual death of the 15K RPM drive. If it also puts to bed the notion of flash as general purpose mass storage, well, I’d be happy to see that as well.

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