Messages in this thread | | | Date | Tue, 17 Jan 2006 17:29:20 -0700 | From | Michael Loftis <> | Subject | Re: FYI: RAID5 unusably unstable through 2.6.14 |
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--On January 17, 2006 7:21:45 PM -0500 Phillip Susi <psusi@cfl.rr.com> wrote:
> Your understanding of statistics leaves something to be desired. As you > add disks the probability of a single failure is grows linearly, but the > probability of double failure grows much more slowly. For example:
What about I said was inaccurate? I never said that it increases exponentially or anything like that, just that it does increase, which you've proven. I was speaking in the case of a RAID-5 set, where the minimum is 3 drives, so every additional drive increases the chance of a double fault condition. Now if we're including mirrors and stripes/etc, then that means we do have to look at the 2 spindle case, but the third spindle and beyond keeps increasing. If you've a 1% failure rate, and you have 100+ drives, chances are pretty good you're going to see a failure. Yes it's a LOT more complicated than that.
> > If 1 disk has a 1/1000 chance of failure, then > 2 disks have a (1/1000)^2 chance of double failure, and > 3 disks have a (1/1000)^2 * 3 chance of double failure > 4 disks have a (1/1000)^2 * 7 chance of double failure > > Thus the probability of double failure on this 4 drive array is ~142 > times less than the odds of a single drive failing. As the probably of a > single drive failing becomes more remote, then the ratio of that > probability to the probability of double fault in the array grows > exponentially. > > ( I think I did that right in my head... will check on a real calculator > later ) > > This is why raid-5 was created: because the array has a much lower > probabiliy of double failure, and thus, data loss, than a single drive. > Then of course, if you are really paranoid, you can go with raid-6 ;) > > > Michael Loftis wrote: >> Absolutely not. The more spindles the more chances of a double failure. >> Simple statistics will mean that unless you have mirrors the more drives >> you add the more chance of two of them (really) failing at once and >> choking the whole system. >> >> That said, there very well could be (are?) cases where md needs to do a >> better job of handling the world unravelling. >> - >
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