RAID 10 and RAID 0+1 offer significantly different failure tolerances.
RAID 0+1:
odds of failure:
RAID0 1/1000000 * 2 = 1/500000 (the RAID0)
RAID1 RAID1 (1/1000)^2 = 1/1000000 (per mirror)
A B C D 1/1000 (per drive)
is better than RAID 10:
RAID1 (1/500)^2 = 1/250000
RAID0 RAID0 1/1000 * 2 = 1/500
A B C D 1/1000
Adding capacity yields:
RAID0 1/1000000 * 3 = 1/333333
RAID1 RAID1 RAID1 (1/1000)^2 = 1/1000000
A B C D E F 1/1000
RAID1 (1/333)^2 = 1/110889
RAID0 RAID0 1/1000 * 3 = 1/333
A B C D E F 1/1000
The failure odds of the RAID0+1 increase slower than the odds for RAID 10
as capacity is added.
IMO, just because the absolute probabilities may be small, the difference
between them (a factor of 2 or more) is still significant. My reasoning
is thus:
I consider the odds of a drive failure to be 100%--not if, but when. The odds
of the system failing then become the odds of the degraded system failing.
IOW, it can always sustain the loss of the first drive--that's the entire
purpose in the first place. So the real issue is how well it handles a
double-fault--how much do you have to worry about getting the first drive
replaced. (If I did this right, the relative odds should be the same--just
their magnitudes increased, but I think it also might make it easier to
visualize.) This makes the odds become:
RAID1-Deg. 1/1000
- B 1/1000
RAID0 1/1000 + 1/1000000 ~= 1/1000
Deg. RAID1 1/1000, 1/1000000 respectively (the latter from above)
- B C D 1/1000
RAID1-Deg. 1/500
_ RAID0 n/a, 1/500
- B C D 1/1000
RAID0 1/1000 + (1/1000000 * 2) ~= 1/1000
Deg. RAID1 RAID1 1/1000, 1/1000000, 1/1000000
- B C D E F 1/1000
RAID1-Deg. 1/333
- RAID0 n/a, 1/1000 * 3 = 1/333
- B C D E F 1/1000
RAID5-Deg. 1/1000 * 3 = 1/333
- B C D 1/1000
RAID5-Deg. 1/1000 * 5 = 1/200
- B C D E F 1/1000
So my final probabilities become:
RAID 1, 2 drives: 1/1,000 = 0.001
RAID 0+1, 4 drives: ~1/1,000 = 0.001
RAID 0+1, 6 drives: ~1/1,000 = 0.001
RAID 10, 4 drives: 1/500 = 0.002
RAID 10, 6 drives: 1/333 = 0.003
RAID 5, 4 drives: 1/333 = 0.003
RAID 5, 6 drives: 1/200 = 0.005
You can see from this that increasing capacity reduces the reliability of a
RAID 0+1 pretty much insignificantly, significantly reduces it for a RAID 10,
but still not as badly as for a RAID5.
> Basically, RAID 1, RAID 10, and RAID 5 differ little in probability of
> failure. For any one of these configurations, you still have to lose
> two drives to fail, and the probability of that is small, as we see
> above. One might compare it to buying four lottery tickets instead of
> one - you're still probably going to lose, even though your chances of
> winning are slightly higher than before.
If you consider as I do, that the first drive failure is a given,
then the odds of failure is magnitudes larger--back up to the same order
of magnitude as a single drive failing.
This makes the relative differences in failure odds become significant,
especially when you consider how the odds increase with additional capacity.
--
Respectfully,
Nicholas Leippe
Sales Team Automation, LLC
1335 West 1650 North, Suite C
Springville, UT 84663 +1 801.853.4090
http://www.salesteamautomation.com
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