Linux: Which process is causing “device busy” when

Question

Linux: Which process is causing "device busy" when doing umount?

Answer 1

Also check /etc/exports. If you are exporting paths within the mountpoint via NFS, it will give this error when trying to unmount and nothing will show up in fuser or lsof.

Answer 2

Check for open loop devices mapped to a file on the filesystem with "losetup -a". They wont show up with either lsof or fuser.

Answer 3

lsof and fuser didn't give me anything either.

After a process of renaming all possible directories to .old and rebooting the system every time after I made changes I found one particular directory (relating to postfix) that was responsible.

It turned out that I had once made a symlink from /var/spool/postfix to /disk2/pers/mail/postfix/varspool in order to minimise disk writes on an SDCARD-based root filesystem (Sheeva Plug).

With this symlink, even after stopping the postfix and dovecot services (both ps aux as well as netstat -tuanp didn't show anything related) I was not able to unmount /disk2/pers.

When I removed the symlink and updated the postfix and dovecot config files to point directly to the new dirs on /disk2/pers/ I was able to successfully stop the services and unmount the directory.

Next time I will look more closely at the output of:

ls -lR /var | grep ^l | grep disk2

The above command will recursively list all symbolic links in a directory tree (here starting at /var) and filter out those names that point to a specific target mount point (here disk2).

Answer 4

Filesystems mounted on the filesystem you're trying to unmount can cause the target is busy error in addition to any files that are in use. (For example when you mount -o bind /dev /mnt/yourmount/dev in order to use chroot there.)

To find which file systems are mounted on the filesystem run the following:

mount | grep '/mnt/yourmount'

To find which files are in use the advice already suggested by others here:

lsof | grep '/mnt/yourmount'

Answer 5

Look at the lsof command (list open files) -- it can tell you which processes are holding what open. Sometimes it's tricky but often something as simple as sudo lsof | grep (your device name here) could do it for you.

Answer 6

That's exactly why the "fuser -m /mount/point" exists.

BTW, I don't think "fuser" or "lsof" will indicate when a resource is held by kernel module, although I don't usually have that issue..