>     file-space's name doesn't strike me as being accurate, it's really
>     "space left for files."  To keep it short, maybe filesystem-space ?

> Hmm, good point.  However, both names have an ambiguity between space in
> use and free space.  How about free-space, since that is really the main
> point?

+1 for `file-free-space`. "Free space" is the term used in many/most
user interfaces.

Common Lisp has a `room` function
(http://www.lispworks.com/documentation/HyperSpec/Body/f_room.htm) but
it displays RAM used, not disk free.