Comment # 17 on bug 931724 from ill lume

"Therefore, what do you suggest?"

Patch the kernel; or have df display an error/warning/message to the user on
btrfs systems that it can't read btrfs space accurately if it detects a btr
filesystem mounted; or since btrfs fs show is more accurate have df get its
numbers from that if possible.

"and simply
not to be taken as-is as for _ALL_ other file systems."

and change the man page to reflect this.

Opensuse 13.2 btrfs implementation is broken right now. On a new install I ran
out of space on a 30GB / partition in a couple of weeks because of snapper, and
I didn't realize it because df said I still had 20GB left.