Diskspace usage

Hello there, I know this question has been asked a lot of time, but my free diskspace has fallen to 2.1G out of 20G, and I really don't know where this space is lost. I ran both du and df commands :
root@ns [~]# df -h Filesystem Size Used Avail Use% Mounted on rootfs 20G 17G 2.1G 90% / /dev/root 20G 17G 2.1G 90% / devtmpfs 16G 460K 16G 1% /dev /dev/md3 1.8T 165G 1.6T 10% /home tmpfs 16G 0 16G 0% /dev/shm /dev/loop0 485M 51M 409M 12% /tmp /dev/loop0 485M 51M 409M 12% /var/tmp

root@ns [~]# du -sh /* | sort -h du: cannot access `/proc/29395/task/29395/fd/4': No such file or directory du: cannot access `/proc/29395/task/29395/fdinfo/4': No such file or directory du: cannot access `/proc/29395/fd/4': No such file or directory du: cannot access `/proc/29395/fdinfo/4': No such file or directory 0 /proc 0 /scripts 0 /sys 4.0K /backup_storage 4.0K /cgroup 4.0K /media 4.0K /mnt 4.0K /razor-agent.log 4.0K /selinux 4.0K /srv 16K /lost+found 16K /quota.user 268K /error_log 460K /dev 7.5M /bin 20M /sbin 27M /lib64 33M /etc 39M /boot 41M /tmp 140M /lib 622M /opt 953M /root 2.0G /var 6.2G /usr 164G /home
As you can see, 17G are used on /, but it seems less space is used according to du command. Do you know how I can effectively find out where my free space is gone? Thank you.