> I run out of disk space the  other day and df output looked a bit wierd:
> 
> dolecek~ > df
> Filesystem  1K-blocks     Used    Avail Capacity  Mounted on
> /dev/wd0a      990582   961902   -20850   102%    /

See /usr/share/doc/smm/05.fastfs/
(McKusick, Joy, Leffler, Fabry:  A Fast File System for UNIX), section 3.

   In order for the layout policies to be effective, a file system
   cannot be kept completely full.  For each file system there is a
   parameter, termed the free space reserve, that gives the minimum
   acceptable percentage of file system blocks that should be free.
   If the number of free blocks drops below this level only the system
   administrator can continue to allocate blocks.

The df command reports the percentage of *available* space, (not the
percentage of *total* space) which is in use.

One nit, though.. the next sentance is an interesting claim:

   The value of this parameter may be changed at any time,
   even when the file system is mounted and active.

.. however, a, umm, documented bug in tunefs prevents this from happening...

					- Bill