BTRFS breathes new air into the old cp command. Suddenly it can be used to create instantaneous copy-on-write (CoW) snapshots of single files via cp --reflink.

When you want to convert a folder to a subvolume, you are best of with the following commands:

# mv folder folder_old
# btrfs su create ./folder
# cp -a --reflink ./folder_old/. folder

If you want to disable data CoW (as with nodatacow mount option [but which isn't that useful because it applies to the whole btrfs volume]) on single files you need apply the xattr +C to it, and defrag, or simply cp it (if it's a folder best to use cp -a folder/. dest).

$ chattr +C file
$ btrfs fi defrag file # or
$ cp --reflink=never file file.new && mv file.new file

And there you have it: cp is suddenly much more useful.