Friday, November 16, 2007

Reed-Solomon libraries

If you want to run a multi-petabyte storage system then you don't want to do it with Raid 5 or Raid 6; with modern disks' ~3% per year failure rate, that's 300 a year when you have 10000 disks and the odds start to get pretty good (relatively speaking) that you'll face permanent data loss at some point when you lose a third disk from an array while two are rebuilding. And of course monitoring and replacing disks in lots of small arrays is manpower-intensive, which to investors translates as "expensive."

You probably don't want to go with triplication, either; disks are cheap, but not so cheap that you want to triple your hardware costs unnecessarily. While storing multiple copies of frequently used data is good, all your data probably isn't "frequently used."

What is the solution? As it turns out, Raid is actually a special case of Reed-Solomon encoding, which lets you specify any degree of redundancy you want. You can be safer than triplication with a fraction of the space needed.

I was prompted to write this because Mozy open-sourced the Reed-Solomon library I used while I was there, librs, complete with Python bindings. The original librs we used at Mozy was written by Byron Clark, a formidible task. Later we switched to the version you see on sourceforge, based on Plank's original encoder. I wasn't involved with librs at all except to fix a couple reference leaks in the Python wrapper.

But if you're actually looking for an rs library to use, Alen Peacock, who is much more knowledgeable than I about the gory details involved here, tells me that if you are starting from scratch the two libraries you should evaluate are zfec, which also comes with Python bindings, and Jerasure which is an updated -- i.e., probably faster than his first -- encoder by Plank. (Jerasure has nothing to do with Java.)