Re: FYI: RAID5 unusably unstable through 2.6.14

[Phillip Susi <psusi@cfl.rr.com>]

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:

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.
> -