On 3/23/06, Robin Bowes <
[EMAIL PROTECTED]> wrote:
Jim Dibb wrote:
> I wasn't referring to write corruption but the chance of simultaneous
> loss of 2 drives. If you have a drive failure in your R1, it still
> needs to be replaced and rebuilt before the second drive also fails.
>
> 2 drive failure is also what you primarily worry about in R5 systems,
> but the chance of two drives failing is larger on R5 because you have
> more drives that could possibly fail.
>
> Once the first drive fails on a R1, the second drive must fail too.
> Once the first drive fails on R5, one of N-1 drives must also fail.
> If the reliability of any drive is .99 for a given time frame, the
> reliability of N of them is (.99)^N. So the second drive in a mirror is
> .99 reliable. The set of 3 other drives in a R5 is (.99)^3 or .97.
...and because all the drives are often around the same age and
operating in the same environment, the other disks in a R5 array can be
"ready to fail" too and the additional load/stress of resyncing after a
drive failure can be enough to tip them over the edge.
Personally, I have a 6-disk R5 array - 4 + 1 parity + 1 hot spare
I'll be going with RAID6 next time - that can withstand the loss of two
drives.
R.
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