Unix df not updating
If you have nfs mounts, try umounting them and looking to see if anything was accidentally written in those directories before the mount.
2) corrupted files: I see this occasionally on windows to linux file transfer via SMB.
Softupdates can also effect filesystem freespace as the actual inode space recovery can be deferred; the sync(8) command can be used to encourage this to happen sooner.
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The prevents the underlying file data from going away while the process is using it.
When the process closes the file if the reference count falls to zero the file space is marked as available.
Its always good practice to null such files instead of deleting them.
This is an annoying problem on a lot of Linux distributions, and it can have several causes.
If this is your situation and you're desperate, you may be able to alter the amount of space reserved for Second, the operating system won't release disk space for deleted files which are still open.
If you've deleted (say) one of Apache's log files, you'll need to restart Apache in order to free the space.
2 others ways to get the disk is full issue: 1) hidden under a mount point: linux will show a full disk with files "hidden" under a mount point.
If you have data written to the drive and mount another filesystem over it, linux correctly notes the disk usage even though you can't see the files under the mount point.
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Dealing with old Cent OS 5.6 box, with no lvm setup, my root file system / is full, I have cleared many old log files and application files that I don't need, which was more then 2 -5GB in size, however my system still reports that disk is full.