[dannyperson]

What happens when a URE is encountered and all the disks are online? It seems that they could be detected early and fixed before a rebuild is necessary by doing a weekly sweep of the entire array, reading every data block.

[harshreality]

With raid 5 if you get a parity block mismatch, you don't know which drive was wrong. You could compare parity bit by bit, to find which bits (it could be anywhere from 1 bit up to the stripe size) was wrong, but you won't be able to figure out how to fix those bit(s) on the stripe without additional information, either a hash or FEC data.

Checksumming filesystems let you find the faulty drive by reconstructing data from each n-choose-(n-1) drive set and finding the set with the correct hash.

Filesystems using FEC instead of raid (5/z1, 6/z2 ...) can also correct data errors, but I'm not aware of any consumer-level filesystems that implement it. I'm not sure why. Doesn't Amazon use it for S3? Data block and FEC data layout on a disk array has to be a solved problem.