`--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.>> ->--"Genius might be described as a supreme capacity for getting its possessorsinto trouble of all kinds."-- Samuel Butler-To unsubscribe from this list: send the line "unsubscribe linux-kernel" inthe body of a message to majordomo@vger.kernel.orgMore majordomo info at  http://vger.kernel.org/majordomo-info.htmlPlease read the FAQ at  http://www.tux.org/lkml/`