df discrepencies

[charlie derr <cderr@simons-rock.edu>]

On one of the machines that I oversee there is an issue with the df output that I don't understand.

here's a part of the output from df -h

/dev/sda1             440G  420G     0 100% /backup

if i don't use the -h it looks like this:

/dev/sda1            461293804 440335112         0 100% /backup

It appears that there really are 20Gigs free, but that column shows 0 -- can i reliably ignore that column and use subtraction with the 
previous two to compute the true free space?


If I clear up enough more diskspace the 4th column will eventually show more than 0, but it won't be equivalent to the difference of the 2nd 
and 3rd.   My recollection is that rebooting will fix this problem (for a while), but I'd prefer not to have to reboot every couple weeks.


This machine is running sarge and that partition is ext3 over a RAID5 array.  I do slip up on occasion and allow this partition to 
completely fill up (not sure if that might be relevant) so that the rsync processes that are writing to it fail (because of no more 
remaining disk space).

	thanks so much for any hints,
		~c