OK then, I guess my next question would be what's the best way to "undedupe"
the data I have?
Would it work for me to zfs send/receive on the same pool (with dedup off),
deleting the old datasets once they have been 'copied'?
yes. Worked for my.
I think I remember reading somewhere that the DDT never shrinks, so this would
not work, but it would be the simplest way.
Once you delete all snapshots with dedup on, the DDT will be empty. This is
what I did here and it worked like a charm. But again: I only had it actively
enabled on two datasets so my situation might be different from yours.
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