https://www.reddit.com/r/btrfs/comments/fcntc6/qcow2_on_btrf...
You can either make a sub volume and mount it with nocow or set the +C file attribute to disable CoW:
http://blog.jim.nz/2015/12/04/btrfs-subvolume-with-nocow.htm...
Doesn’t really look like nilfs2 has this kind of flexibility, though