Why does "cp -R --reflink=always" perform a standard copy on a Btrfs filesystem?

Btrfs support Copy-On-Write. I tried to use that feature to clone a directory: cp -R --reflink=always foo_directory foo_directory.mirror I expected the command to finish almost instantly (like a btrfs subvolume snapshot), but the cp command seems to perform a slow, standard copy. According to the man page, I would expected --reflink=always to enforce Copy-On-Write: > When --reflink[=always] is specified, perform a lightweight copy, where the data blocks are copied only when modified. If this is not possible the copy fails, or if --reflink=auto is specified, fall back to a standard copy. **Questions:** - Do you know why --reflink=always doesn't work? - What options (or other commands) should I use instead?