df seems to be reporting free space incorrectly

Christopher Miner <[email protected]>
Newsgroups gmane.linux.redhat.release.guinness
Message-ID <[email protected]>
A server running Red Hat 7.0 just started acting strange. I'm fairly new to linux administration and I've had this same problem twice on the same server. The df command was reporting that the / partition was only using 128Mb of file space. All the sudden, it started reporting that the / partition (6Gb) is completely full. I can't unmount any devices because it says it can't write to the mtab.tmp file. I can't run kde or gnome because xinit won't start. When I type mail, it says "/tmp: no space left of this device", but the /tmp dir is virtually empty. I ran du and totaled the disk usage and noticed there is about a 5.8-5.9 Gb difference between what du and df are reporting. 
I have no idea why this problem is happening or how to fix it (short of formatting / and /usr partitions and reinstalling linux). Any help???
Chris Miner
[email protected]
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