Daniel,
When we take into account time, the models we use are Markov models
which consider the amount of space used, disk size, block and whole
disk failures, RAID scheme, recovery from tape time, spares, etc.
All of these views of the system are being analyzed. Needless to say,
with the zillions of permutations of RAID schemes possible with a
system such as the Sun Fire X4500, we'll never model all of them.
Hence my request for the popular configs which I will model in detail.
-- richard
Daniel Rock wrote:
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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