what do you mean it breaks? does it purge* the older versions, or does something break?

* (yes, DEC reference)

It deduplicates everything. Don't defrag Btrfs lightly, unless you have a specific issue to solve and you ran out off other solutions. Even then, only do it if you know what you are doing.

*reduplicates

Anyway, deduplication can be recovered after a defrag by running duperemove or similar tools. But it's a lot of added work so it's only really worth it if fragmentation is an issue in the first place.

And you have to cross your fingers and hope it picks the defragmented copy, as far as I know.

It's also very slow to go through that on multiple snapshots.

And I think some of the metadata doesn't like to recombine?