Right, but to then compare the memory use of ZFS with dedupe turned on against another fs without that feature would be disingenuous since dedupe is optional in ZFS. That was the whole point of my original comment.

deduplication doesn't have necessarily to be expensive; HAMMER (the first version, not HAMMER2) has offline deduplication that is usually scheduled to run at night. This allows regular use to be quite performant and with a low memory footprint. Of course there are tradeoffs, in particular heavy disk usage at certain hours (which can be an issue depending on the workload) and the fact that space is reclaimed only after some time that it has been wasted.

I did say that ZFS metadata in general, and dedupe especially is much bigger than it needs to be. Even live dedupe can be done much more cheaply than how ZFS does it.