If you truncate, write new data, and close fd, nothing (in that file) refers to the old extents, and btrfs will garbage collect them just fine. The thing that

If you truncate, write new data, and close fd, nothing (in that file) refers to the old extents, and btrfs will garbage collect them just fine. The thing that hurts all copy-on-write systems is something like SQLite/Postgres/etc that mutates the file here and there; explicitly not overwriting it. chattr +C is the standard convention to opt out of copy-on-write, for those.