Delayed reaction

My understanding is that COW is fundamental to how ZFS works (refer back to my previous comment on Merkle trees and how data and meta-data reference each other). I havn't read the code to verify this, but it's what all the papers and presentations on ZFS say.

I think that also already answers your question as to how ZFS can know which one is correct. I'm not sure there's any value in, again, explaining how ZFS consistency (in theory) comes down to the update of a single "uber-block".