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I just did something similar to this:

$ du -sh /bigdata/stuff
1022G /bigdata/stuff

$ df -h /bigdata
Fileystem  Size  Used  Avail  Used% mounted on
/dev/sda5  1,8T  1,7T  140G   93%   /bigdata

$ rm -rf /bigdata/stuff/*

$ df -h /bigdata
Fileystem  Size  Used  Avail  Used% mounted on
/dev/sda5  1,8T  1,6T  268G   86%   /bigdata

which means that deleting 1022 G of files only gave me 128 G of additional free space.
There are no further filesystems mounted under /bigdata and there were no open files on /bigdata at this moment.

In order to reclaim all of the free space, I needed to umount /bigdata (taking v...e...r...y long) and mount it again. After that, everything was back to normal.
According to fsck, there are no filesystem errors (fsck was performed only for error checking, it is not needed for reclaiming the space).
According to a S.M.A.R.T. extended self test, the harddisk does not have hardware errors.

Does anybody know what's wrong ?

System info:
I am using ext4 on Ubuntu Server 12.04 64bit on a harddisk bought a few months ago, so there are no inherited problems from a previous installation (as there is none). The filesystem was created using -m 0 (0% reserved for root).

History:
I already noticed this problem a few years ago (on ext3). Up to now, I thought it only happens when I access /bigdata via NFS and do the operation from the client. But now, I have seen that it also happens when deleting files locally.

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Is any process keeping a file open when you're deleting them? If so, the blocks on the file system will not be free until that process actually releases them. You can use lsof to check this. –  Jenny D Apr 18 '13 at 14:41
    
No, definitely no open files. Otherwise I could not do umount. –  Markus N. Apr 18 '13 at 14:51
    
Next time, check that before starting to delete the files, and definitely before doing umount. –  Jenny D Apr 18 '13 at 14:54
    
Try a sync to force the update without unmounting. –  Bob Apr 18 '13 at 15:00
    
I already tried this, but forgot to mention it in my question –  Markus N. Apr 18 '13 at 15:07

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