I often experience a problem to umount a directory.

umount /mnt/dir
umount: /mnt/dir: device is busy

There are many reasons why the device is busy. Sometimes there are processes running which have open locks on it, sometimes there are other directories mounted on top of /mnt/dir .

My question.

What are the steps to check why a directory couldn't be unmounted.

I know there are many reasons, but it's ok if you explain a specific solution.


[X] running processes on mounted volumes.
[X] another volume is mounted on top of a volume we want to unmount
[_] NFS locks the volume we want to unmount

Best Answer

The way to check is fuser -vm /mnt/dir , which must be run as root. It will tell you which processes are accessing the mount point.

An alternative is lsof /mnt/dir , which will show each open file on the mount. Again best run as root.

You can run either of these as non-root, but then the output will be limited to your processes—ones from other users will just be silently not shown, even though they will prevent unmounting the filesystem.


Watt:~# fuser -vm /mnt/Zia/src
                     USER        PID ACCESS COMMAND
/mnt/Zia/src:        root     kernel mount /mnt/Zia/src
                     anthony   24909 ..c.. bash
                     anthony   25041 F.c.. gvim

The field of access tells you how it is accessed In this case, the kernel has it in use as a mount (duh, but unmount will be ok with only this). bash has it as the current working directory (will have to cd to a different directory before unmount) and gvim both has the current directory and has a file open (will need to close that gvim).

Watt:~# lsof /mnt/Zia/src
bash    24909 anthony  cwd    DIR   0,26    12288 3527682 /mnt/Zia/src/perl (zia.vpn.home:/home/anthony/src)
gvim    25041 anthony  cwd    DIR   0,26    12288 3527682 /mnt/Zia/src/perl (zia.vpn.home:/home/anthony/src)
gvim    25041 anthony    6u   REG   0,26    16384 3526219 /mnt/Zia/src/perl/.utf8.c.swp (zia.vpn.home:/home/anthony/src)

In this output, you can see the current directories for both bash and gvim (as type DIR ). You can also see which file gvim has open for write.

How to force the issue:

fuser has a -k option which will send a signal (default: SIGKILL ) to each process using the mount. It's a fairly strong way to stop the mount from being busy (And of course, be careful of what you SIGKILL !)

umount has an -l option to perform a lazy unmount. The mount will be removed from the filesystem namespace (so you won't see it under /mnt/Zia/src anymore, in the example) but it stays mounted, so programs accessing it can continue to do so. When the last program accessing it exits, the unmount will actually occur.

There is one final fixable cause of unmount failing, and that's an NFS server going down. Here you can use umount -f , but you risk data loss if you do so. (The client may have cached writes that haven't been confirmed by the server yet, and those writes will be discarded. Apps, however, have already been told the write is successful.)