Raid 5

Corey Edwards tensai at zmonkey.org
Fri Dec 2 11:54:05 MST 2005


On Fri, 2005-12-02 at 11:10 -0700, Tyler Strickland wrote:
> In conclusion, here are the final probabilities:
> RAID 5, 4 drives:   6/1,000,000 = 0.000006
> RAID 1, 2 drives:   1/1,000,000 = 0.000001
> RAID 10, 4 drives:  2/1,000,000 = 0.000002
> RAID 10, 6 drives:  3/1,000,000 = 0.000006

Carrying that further out I think the differences become even clearer.

RAID 5, 6 drives:   0.000015
RAID 10, 6 drives:  0.000003
RAID 5, 8 drives:   0.000028
RAID 10, 8 drives:  0.000004
RAID 5, 10 drives:  0.000045
RAID 10, 10 drives: 0.000005

Which makes complete sense because if you have 1 dead disk and another
one fails, in a RAID 10 with 10 drives you have a 1 in 9 chance of it
being the one disk that will kill your volume. And 0.000045 / 0.000005 =
9/1, or 9 times more likely that a RAID 5 will die than a RAID 10.

When you take sizes into consideration, a RAID 5 w/ 6 disks is the same
size as a RAID 10 w/ 10 disks. Even in that situation, the RAID 5 is 3
times more likely to die, and the RAID 10 having more spindles will
theoretically be faster.

Thanks for putting my words into numbers.

Corey

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