Home / server / Questions / 880265
In Process
Some Linux Nerd
Asked: 2017-10-26 08:48:59 +0800 CST

How do I unmount one of two devices mounted to the same mount point?

  • 772

I accidentally left two different devices mounted on /opt:

/dev/xvdf on /opt type ext4 (rw,relatime,seclabel,data=ordered)
/dev/md0 on /opt type ext4 (rw,relatime,seclabel,stripe=256,data=ordered)

It would be a problem to umount /dev/md0, and things are using /opt. /dev/md0 should be mounted on top of /dev/xvdf. Any suggestions how I can just unmount /dev/xvdf?

Tried:

[root@redacted ~]# umount /dev/xvdf
umount: /dev/xvdf: umount failed: Invalid argument

[root@redacted ~]# mount --move /dev/xvdf /temp
mount: bad option. Note that moving a mount residing under a shared
       mount is unsupported.

Also I've had AWS support for volumes yell at me about force detaching in-use volumes, so that's not an option.

linux

3 Answers

  1. You cannot do it atomically. You can however do it with a sequence of mount --move commands. And you will need two other directories to use as mount points.

    cp /etc/mtab /root/mtab-before
mkdir /mnt/shuffle-md0 /mnt/shuffle-xvdf
mount --move /opt /mnt/shuffle-md0
mount --move /opt /mnt/shuffle-xvdf
mount --move /mnt/shuffle-md0 /opt
umount /mnt/shuffle-xvdf
cp /etc/mtab /root/mtab-after

    Notice that the /etc/mtab entry for /dev/xvdf may end up looking pretty weird in the end. So I recommend you create a copy of /etc/mtab before you start such that you can reconstruct that entry once you are done.

    Anything opening paths through /opt while you are shuffling around the mount points may get unexpected results. But files and directories which were opened before you started will be unaffected by this maneuver.

    • 4

  2. @kasperd solution didn't work for me, because I've received the message:

    It is forbidden to move a mount residing under a shared mount

    The easy solution is to comment out the offending mount point in file /etc/fstab, and reboot the server.

    • 1

  3. You should be able to run:

    umount /dev/xvdf /opt
    • 0