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.
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