Linux missing disk space

I have KVM vps with strange disk usage:

# df -h
Filesystem            Size  Used Avail Use% Mounted on

/dev/sdb              493G  1.2G  466G   1% /
tmpfs                 4.0G     0  4.0G   0% /dev/shm
/dev/sda1              96M   41M   51M  45% /boot

# du -sh /
du: cannot access `/proc/1633/task/1633/fd/4': No such file or directory
du: cannot access `/proc/1633/task/1633/fdinfo/4': No such file or directory
du: cannot access `/proc/1633/fd/4': No such file or directory
du: cannot access `/proc/1633/fdinfo/4': No such file or directory
1021M   /

How could it be? Where are ~20G of free space?

Answer

I guess you talk about the discrepancy between 493GB total space, 1.2G used and 466GB free. This is likely the result of the usual 5% of disk space that is reserved for root and not generally available.

To check for this, please add the output of at least the Reserved block count from

tune2fs -l /dev/sdb 

to your question (provided you use ext3 or ext4 as a filesystem)