The calculation of RAID 6 failure rate is wrong

[harleylg]

RAID 5 will failed on URE while rebuilding is because there's no enough redundant information to rebuild the strip. But RAID 6 had enough.

Imaging there is a 4+1 RAID 5, when it's health, we could repaired URE by scrubbing the array, recalculate the write data on the rust sector and rewrite it to that sector, unless there's two or more URE in one strip which chance we could ignore.

So when a 4+2 RAID 2 with one disk failure. It equivalent a 4+1 RAID 5. So if an URE happened during the rebuild progress, it could be fixed and does not influences the rebuild progress.