On Fri, 18 Jan 2013, Stan Hoeppner wrote:
If my math is correct, with a URE rate of 10E14, that's one URE for every ~12.5TB read. So theoretically one would have to read the entire 2TB drive more than 6 times before hitting the first URE. So it seems unlikely that one would hit a URE during a mirror rebuild with such a 2TB drive.
Unlikely yes, but it also means one in 6 rebuilds (statistically) will fail with URE. I'm not willing to take that chance, thus I use RAID6.
Usually, with scrubbing etc I'd imagine that the probability is better than 1 in 6, but it's still a substantial risk.
This is math so there is no room for disagreement--there is one right answer. Either mine is correct or yours is. If my math is incorrect I'd certainly appreciate it if you, or anyone else, would explain where I'm in error, so I don't disseminate incorrect information in the future. But given that the articles I've read on this subject agree with my math, I don't believe I'm in error.
With a BER of 10^-14 you have a 16% risk of getting URE when reading an entire 2TB drive. We both agree on that. You called the risk in comparison with "getting hit by lightning" which I strongly disagree with.
I made the point in a previous post that I use the smallest drives I can get away with for a given array/workload/capacity, as performance is generally better and rebuild times much lower. Potential URE issues provide yet another reason to use a higher count of smaller drives, though again, this doesn't tend to affect most RAID1/10 users, yet.
It's important that people make informed decsision. With enterprise drives (better BER) that are smaller (let's say 300 GB), the risk is greatly reduced.
-- Mikael Abrahamsson email: swmike@xxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
