Because:

> When dedup is enabled [...] every single write and free operation requires a lookup and a then a write to the dedup table, regardless of whether or not the write or free proper was actually done by the pool.

To me, this is "obviously" the wrong approach in most cases. When I'm writing data, I want that write to complete as fast as possible, even at the cost of disk space. That's why I don't save files I'm actively working on in 7zip archives.

But later on, when the system is quiet, I would love for ZFS to go back and figure out which data is duplicated, and use the BRT or whatever to reclaim space. This could be part of a normal scrub operation.