Linux: Which process is causing “device busy” when doing umount? [closed]

Answer 1

You should use the fuser command.

Eg. fuser /dev/cdrom will return the pid(s) of the process using /dev/cdrom.

If you are trying to unmount, you can kill theses process using the -k switch (see man fuser).

Answer 2

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 3

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 4

lsof +f -- /mountpoint

(as lists the processes using files on the mount mounted at /mountpoint. Particularly useful for finding which process(es) are using a mounted USB stick or CD/DVD.

Answer 5

Open files

Processes with open files are the usual culprits. Display them:

lsof +f -- <mountpoint or device>

There is an advantage to using /dev/<device> rather than /mountpoint: a mountpoint will disappear after an umount -l, or it may be hidden by an overlaid mount.

fuser can also be used, but to my mind lsof has a more useful output. However fuser is useful when it comes to killing the processes causing your dramas so you can get on with your life.

List files on <mountpoint> (see caveat above):

fuser -vmM <mountpoint>

Interactively kill only processes with files open for writing:

fuser -vmMkiw <mountpoint>

After remounting read-only (mount -o remount,ro <mountpoint>), it is safe(r) to kill all remaining processes:

fuser -vmMk <mountpoint>

Mountpoints

The culprit can be the kernel itself. Another filesystem mounted on the filesystem you are trying to umount will cause grief. Check with:

mount | grep <mountpoint>/

For loopback mounts, also check the output of:

losetup -la

Anonymous inodes (Linux)

Anonymous inodes can be created by:

• Temporary files (open with O_TMPFILE)
• inotify watches
• [eventfd]
• [eventpoll]
• [timerfd]

These are the most elusive type of pokemon, and appear in lsof's TYPE column as a_inode (which is undocumented in the lsof man page).

They won't appear in lsof +f -- /dev/<device>, so you'll need to:

lsof | grep a_inode

For killing processes holding anonymous inodes, see: List current inotify watches (pathname, PID).

Answer 6

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.