# 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   /

怎么会这样？ 〜20G的可用空间在哪里？