Richard Elling schrieb:
Jeff Bonwick wrote:
For 6 disks, 3x2-way RAID-1+0 offers better resiliency than RAID-Z
or RAID-Z2.
Maybe I'm missing something, but it ought to be the other way around.
With 6 disks, RAID-Z2 can tolerate any two disk failures, whereas
for 3x2-way mirroring, of the (6 choose 2) = 6*5/2 = 15 possible
two-disk failure scenarios, three of them are fatal.
For the 6-disk case, with RAID-1+0 you get 27/64 surviving states
versus 22/64 for RAID-Z2. This accounts for the cases where you could
lose 3 disks and survive with RAID-1+0.
I think this type of calculation is flawed. Disk failures are rare and
multiple disk failures at the same time are even more rare.
Let's do some other calculation:
1. Assume each disk reliability independent of the others.
For ease of calculation:
2. One week between disk failure and its replacement (including resilvering)
3. Failure rate of 1% per week for each disk.
Compare:
a. 6 disk RAID-1+0
b. 6 disk RAID-Z2
i. 1 disk failures have a probability of
~5.7 % per week
but more interesting:
ii. 2 disk failures
0.14 % probability per week
a. fatal probability: 20%
b. fatal probability: 0%
iii. 3 disk failures
0.002% probability per week
a. fatal probability: 60%
b. fatal probability: 100%
The remaining probabilites become more and more unlikely.
In summary:
Probability for a fatal loss
a. 0.14% * 20% + 0.002% * 60% = 0.03% per week
b. 0.14% * 0% + 0.002% * 100% = 0.002% per week
Daniel
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