disk usage

[me-5yx05kfkO/aqeI1yJSURBw at public.gmane.org (Matthew Gillen)]

I'm stumped by my home fileserver's root filesystem usage.  I can't understand
why it's full.  df -h reports this:

$ df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/RootFsPrimary-RootPrimary
                       30G   28G  460M  99% /

If I mount /dev/mapper/RootFsPrimary-RootPrimary somewhere else and cd to it,
then run du -sh *, I get this (I deleted all the directories that were only 4K):
7.0M	bin
12K	dev
102M	etc
98M	lib
26M	lib64
20K	lost+found
12K	mnt
24M	root
17M	sbin
68K	tmp
4.9G	usr
1.1G	var

That adds up to less than 7G.  Why is my filesystem full then?  Where did that
other 21G go?  Any ideas?

Thanks,
Matt