某些时候由于nfs服务卡住或者网络问题导致umount -f也无法卸载时，可以临时使用umount -l模式完成卸载

-l, --lazy
Lazy unmount. Detach the filesystem from the filesystem hierarchy now, and cleanup all references to the filesystem as soon as it is not busy anymore. (Requires kernel 2.4.11 or
later.)

A system reboot would be expected in near future if you're going to use this option for network filesystem or local filesystem with submounts. The recommended use-case for umount
-l is to prevent hangs on shutdown due to an unreachable network share where a normal umount will hang due to a downed server or a network partition. Remounts of the share will not
be possible.