linux 服務器掛載了一個nfs路徑192.168.0.12:/backup 做備份，後來192.168.0.12關閉了，但服務器沒有先進行umont.
於是進入Linux服務器，使用df -h 就沒有反應了。
先使用了 umount -f 192.168.0.12:/backup 一點效果也沒有
使用fl參數，
umount -fl 192.168.0.12:/backup ，終於有了效果。
兩個參數的含義：
-f, --force
Force an unmount (in case of an unreachable NFS system).
Note that this option does not guarantee that umount command does not hang. It's strongly recommended to use absolute paths without symlinks to avoid unwanted readlink and stat system calls on unreachable NFS in umount.
-l, --lazy
Lazy unmount.
Detach the filesystem from the file hierarchy now, and clean up all references to this filesystem as soon as it is not busy anymore.