Disk Usage

[Michael Conlen]

I have a NFS server running FreeBSD-4.9-RELEASE. It's run fine for 
several months with five FreeBSD 4.9 systems mounting it's filesystems. 
Suddenly something started using disk space at the rate of 10 GB/hour 
on one of the filesystems (which has exported directories). The catch 
is that a du -k shows a total usage for that file system of much less 
than df -k. du -k essentially shows the disk usage before the available 
space started to disappear! Normally I'd presume someone's hiding files 
under a mount point when I see this but nothings mounted on a directory 
in this filesystem. Upon reboot the space is not used anymore, df -k 
and du -k report similar values.

Quite simply odd. Some other details... ...this has happened twice in 
one day, and the rate of "ghost" disk usage is constant and identical 
in both graphs. The file server is used to serve files to clustered web 
servers. There's considerable write activity to the NFS server all the 
time (40-60Mbit/sec) and moderate read access (~10Mbit/sec).

Any ideas what would cause the df -k and du -k discrepancy?

--
Michael Conlen