On Wed, Nov 12, 2008 at 08:53:46AM -0500, Jeff Soules wrote: > On Wed, Nov 12, 2008 at 3:44 AM, lee <[EMAIL PROTECTED]> wrote: > > Do you mean it is more likely that any one drive in the array fails when > > you have more drives, or do you mean that it is more likely for a drive > > in the array to fail when you have more drives? If drives fail more > > often when being used in an array with more drives, what makes them > > fail more often under those conditions? > > It's purely a statistical property, not related to being in a RAID > array. But if there's (say) a 5% chance for a given drive to fail on > a given day, there's a 95% chance it won't fail. > If you have two drives, the chance *both* won't fail is the chance of > one not failing, times the chance of the other not failing -- 95% > times 95%, or 90.25%. > > With 24, the chance of all the drives not failing is .95^24 or 29.2%. > > Of course I just made the rates up, the survival chances of individual > drives are higher. But logic holds; the more drives you're watching, > the more lucky you'd have to be for none of them to be a dud. > Jeff, you math is off - way off.
P(one fails) != 5/100 P(two drives fail at the same time) = P(one fails) * P(one fails) = 25/10000 If you have more than 2 drives in the raid you have to make the cobinatoric calculations of how many configuration can be there for two drives out of n. that would be 2! * (n-2)! / n! multiply that to P( two drives fail at the same time) where n is the number of all drives. Henning -- Henning Follmann | [EMAIL PROTECTED] it consultant | www.itcfollmann.com -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of "unsubscribe". Trouble? Contact [EMAIL PROTECTED]
