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Scrub/verify/patrols, whatever you want to call it, with RAID all it can do is say, "Well shit, these two copies don't match. What do you want me to do about it, boss?"

ZFS doesn't have to guess which copy is wrong. It knows, and it will automatically replace it.

More, ZFS will even do this on a ZFS mirror when reading half the data blocks from one disk and half from the other, because it reads the cryptographically-strong checksums in with each data block and checks them before delivering the data to the application. If the checksum doesn't match, it rewrites that block from the redundant copy on the other disk(s).

RAID can't do that. If one of a mirror's data blocks is corrupted on disk but with a correct ECC, so that the two blocks don't match but both read cleanly, RAID can't tell which one is correct, so it'll typically just force the system administrator to choose one disk to overwrite the other with. That exchanges astronomical odds against incorrect data for coin flip odds against.




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