The tool du does not handle btrfs volumes correctly. The total reported on my
system is almost twice the volume capacity. This is unexpected and should be
indicated with a warning, both in the output and in the documentation. Since
the head of the output is often lost because it is not that important, the
warning should be emitted at the end.
The total reported by { du /; } agrees on the total reported by { btrfs
filesystem du /; } but the latter command also prints a shared size which is
not paradoxical. The problem with the tool btrfs is that it is a system
command and not a user command so our code cannot just execute their code and
be happy with it. Also, their output is different because it has 3 columns as
I mentioned earlier.
I understand that the total reported by { du /; } is still useful if one were
to create an archive of the file system; such an archive would be much bigger
than the file system itself and thus could not be restored unless the volume
has lots of free space or the archiver itself is btrfs-aware, which I am
afraid it is not. The moral of this story apparently is that btrfs volumes
cannot be reasonably archived using standard tools.
