Wednesday Jul 23, 2008

Hybrid Storage Pools: The L2ARC

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!

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Adam Leventhal, Fishworks engineer

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