On Friday 02 December 2005 05:22 pm, Shane Hathaway wrote: > While your point stands, the calculation is imprecise. By this logic > (1/1000 * 2 = 1/500), flipping a coin twice guarantees the coin will > land on both sides, which observation disproves: > > 1/2 * 2 = 1 >
Yes, my stats couldn't be rustier. I'm glad it was at least a close approximation. I'm surprised Steve Meyers didn't catch me first. ;) > However, your method yields a close approximation for small fractions. > A simple but correct way to compute the reliability of a RAID 0 array is > to subtract the probability of each drive failing from 1, yielding the > probability of each drive surviving, then multiply those probabilities > together to figure out the probability of the set surviving, then > subtract from 1 again to figure out the probability of the set failing. > IOW: > > 1 - (1 - 1/1000) ^ 2 = 1999 / 1000000 > > ... which is really close to 1/500. The difference matters when you > talk about a longer period of time than the 1/1000 estimate implies. > > BTW, has anyone tried RAID 6? It's in recent Linux kernels. It claims > to survive the loss of any two drives in a set. > I have been very curious to know more about raid 6 performance. From my limited knowledge of it, it can't perform better than raid 5 for writes. (I wouldn't think that reads would be impacted any significant amount). But with the added fault tolerance, it may be well worth it. > If anyone's interested, I've written about storage reliability in my > blog, although I'm only discussing theory, not practice. > > http://hathawaymix.org/Weblog/2005-10-26 That's a nice write up. Thanks. -- Respectfully, Nicholas Leippe Sales Team Automation, LLC 1335 West 1650 North, Suite C Springville, UT 84663 +1 801.853.4090 http://www.salesteamautomation.com /* PLUG: http://plug.org, #utah on irc.freenode.net Unsubscribe: http://plug.org/mailman/options/plug Don't fear the penguin. */
