(I'm not an expert, just a user) Snapshotting effectively disables NOCOW. Snapshotting always works, and needs COW to do so. Which means ryao is right that fragmentation will occur. btrfs doesn't copy a NOCOW file in its entirety if it's in a snapshot, or I would have run out of disk space by now. (I really need to move my VM images to a non-backed-up subvolume.)