Returns the current disk usage as a value between 0.0 and 1.0 by
        dividing :attr:`usage` by 100.

def value(self):
        """
        Returns the current disk usage as a value between 0.0 and 1.0 by
        dividing :attr:`usage` by 100.
        """
        # This slightly convoluted calculation is equivalent to df's "Use%";
        # it calculates the percentage of FS usage as a proportion of the
        # space available to *non-root users*. Technically this means it can
        # exceed 100% (when FS is filled to the point that only root can write
        # to it), hence the clamp.
        vfs = os.statvfs(self.filesystem)
        used = vfs.f_blocks - vfs.f_bfree
        total = used + vfs.f_bavail
        return min(1.0, used / total)