I find myself needing to rearrange a system's partitions to move data previously under the root filesystem into dedicated mount points. The volumes are all in LVM, so this is relatively easy: create new volumes, move data into them, shrink the root filesystem, then mount the new volumes at the appropriate points.

The issue is step 3, shrinking the root filesystem. The filesystems involved are ext4, so online resizing is supported; however, while mounted, the filesystems can only be grown. To shrink the partition requires unmounting it, which of course is not possible for the root partition in normal operation.

Answers around the Web seem to revolve around booting a LiveCD or other rescue media, doing the shrink operation, then booting back into the installed system. However, the system in question is remote, and I have access only via SSH. I can reboot, but booting a rescue disc and doing operations from the console is not possible.

How can I unmount the root filesystem while maintaining remote shell access?

  • Any opportunity to temporarily mount the root filesystem on a different server ? e.g. spin up another VM and present this disk volume to it ? – steve Sep 1 '15 at 21:37
  • The server is physical, so no. – Tom Hunt Sep 1 '15 at 21:40
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    Copy root to tmpfs and pivot_root onto there. An example here dreamlayers.blogspot.co.uk/2012/10/running-linux-from-ram.html - it tricky but if you have a test box to try it on, worth considering. – steve Sep 1 '15 at 21:42
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    Another example here, where remote access via ssh is considered ivarch.com/blogs/oss/2007/01/… – steve Sep 1 '15 at 21:51
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    If the root LVM is small enough you can clone it to another LVM and create boot eatery (temp new default) in grub to use it, and then boot from it (making it your "live system") – Rabin Sep 1 '15 at 22:23
up vote 115 down vote accepted

In solving this issue, the information provided at http://www.ivarch.com/blogs/oss/2007/01/resize-a-live-root-fs-a-howto.shtml was pivotal. However, that guide is for a very old version of RHEL, and various information was obsolete.

The instructions below are crafted to work with CentOS 7, but they should be easily enough transferable to any distro that runs systemd. All commands are run as root.

  1. Ensure the system is in a stable state

    Make sure no one else is using it and nothing else important is going on. It's probably a good idea to stop service-providing units like httpd or ftpd, just to ensure external connections don't disrupt things in the middle.

    systemctl stop httpd
    systemctl stop nfs-server
    # and so on....
    
  2. Unmount all unused filesystems

    umount -a
    

    This will print a number of 'Target is busy' warnings, for the root volume itself and for various temporary/system FSs. These can be ignored for the moment. What's important is that no on-disk filesystems remain mounted, except the root filesystem itself. Verify this:

    # mount alone provides the info, but column makes it possible to read
    mount | column -t
    

    If you see any on-disk filesystems still mounted, then something is still running that shouldn't be. Check what it is using fuser:

    # if necessary:
    yum install psmisc
    # then:
    fuser -vm <mountpoint>
    systemctl stop <whatever>
    umount -a
    # repeat as required...
    
  3. Make the temporary root

    mkdir /tmp/tmproot
    mount -t tmpfs none /tmp/tmproot
    mkdir /tmp/tmproot/{proc,sys,dev,run,usr,var,tmp,oldroot}
    cp -ax /{bin,etc,mnt,sbin,lib,lib64} /tmp/tmproot/
    cp -ax /usr/{bin,sbin,lib,lib64} /tmp/tmproot/usr/
    cp -ax /var/{account,empty,lib,local,lock,nis,opt,preserve,run,spool,tmp,yp} /tmp/tmproot/var/
    

    This creates a very minimal root system, which breaks (among other things) manpage viewing (no /usr/share), user-level customizations (no /root or /home) and so forth. This is intentional, as it constitutes encouragement not to stay in such a jury-rigged root system any longer than necessary.

    At this point you should also ensure that all the necessary software is installed, as it will also assuredly break the package manager. Glance through all the steps, and make sure you have the necessary executables.

  4. Pivot into the root

    mount --make-rprivate / # necessary for pivot_root to work
    pivot_root /tmp/tmproot /tmp/tmproot/oldroot
    for i in dev proc sys run; do mount --move /oldroot/$i /$i; done
    

    systemd causes mounts to allow subtree sharing by default (as with mount --make-shared), and this causes pivot_root to fail. Hence, we turn this off globally with mount --make-rprivate /. System and temporary filesystems are moved wholesale into the new root. This is necessary to make it work at all; the sockets for communication with systemd, among other things, live in /run, and so there's no way to make running processes close it.

  5. Ensure remote access survived the changeover

    systemctl restart sshd
    systemctl status sshd
    

    After restarting sshd, ensure that you can get in, by opening another terminal and connecting to the machine again via ssh. If you can't, fix the problem before moving on.

    Once you've verified you can connect in again, exit the shell you're currently using and reconnect. This allows the remaining forked sshd to exit and ensures the new one isn't holding /oldroot.

  6. Close everything still using the old root

    fuser -vm /oldroot
    

    This will print a list of processes still holding onto the old root directory. On my system, it looked like this:

                 USER        PID ACCESS COMMAND
    /oldroot:    root     kernel mount /oldroot
                 root          1 ...e. systemd
                 root        549 ...e. systemd-journal
                 root        563 ...e. lvmetad
                 root        581 f..e. systemd-udevd
                 root        700 F..e. auditd
                 root        723 ...e. NetworkManager
                 root        727 ...e. irqbalance
                 root        730 F..e. tuned
                 root        736 ...e. smartd
                 root        737 F..e. rsyslogd
                 root        741 ...e. abrtd
                 chrony      742 ...e. chronyd
                 root        743 ...e. abrt-watch-log
                 libstoragemgmt    745 ...e. lsmd
                 root        746 ...e. systemd-logind
                 dbus        747 ...e. dbus-daemon
                 root        753 ..ce. atd
                 root        754 ...e. crond
                 root        770 ...e. agetty
                 polkitd     782 ...e. polkitd
                 root       1682 F.ce. master
                 postfix    1714 ..ce. qmgr
                 postfix   12658 ..ce. pickup
    

    You need to deal with each one of these processes before you can unmount /oldroot. The brute-force approach is simply kill $PID for each, but this can break things. To do it more softly:

    systemctl | grep running
    

    This creates a list of running services. You should be able to correlate this with the list of processes holding /oldroot, then issue systemctl restart for each of them. Some services will refuse to come up in the temporary root and enter a failed state; these don't really matter for the moment.

    If the root drive you want to resize is an LVM drive, you may also need to restart some other running services, even if they do not show up in the list created by fuser -vm /oldroot. If you find you are unable to resize an LVM drive under Step 7, try systemctl restart systemd-udevd.

    Some processes can't be dealt with via simple systemctl restart. For me these included auditd (which doesn't like to be killed via systemctl, and so just wanted a kill -15). These can be dealt with individually.

    The last process you'll find, usually, is systemd itself. For this, run systemctl daemon-reexec.

    Once you're done, the table should look like this:

                 USER        PID ACCESS COMMAND
    /oldroot:    root     kernel mount /oldroot
    
  7. Unmount the old root

    umount /oldroot
    

    At this point, you can carry out whatever manipulations you require. The original question needed a simple resize2fs invocation, but you can do whatever you want here; one other use case is transferring the root filesystem from a simple partition to LVM/RAID/whatever.

  8. Pivot the root back

    mount <blockdev> /oldroot
    mount --make-rprivate / # again
    pivot_root /oldroot /oldroot/tmp/tmproot
    for i in dev proc sys run; do mount --move /tmp/tmproot/$i /$i; done
    

    This is a straightforward reversal of step 4.

  9. Dispose of the temporary root

    Repeat steps 5 and 6, except using /tmp/tmproot in place of /oldroot. Then:

    umount /tmp/tmproot
    rmdir /tmp/tmproot
    

    Since it's a tmpfs, at this point the temporary root dissolves into the ether, never to be seen again.

  10. Put things back in their places

    Mount filesystems again:

    mount -a
    

    At this point, you should also update /etc/fstab and grub.cfg in accordance with any adjustments you made during step 7.

    Restart any failed services:

    systemctl | grep failed
    systemctl restart <whatever>
    

    Allow shared subtrees again:

    mount --make-rshared /
    

    Start the stopped service units - you can use this single command:

    systemctl isolate default.target
    

And you're done.

Many thanks to Andrew Wood, who worked out this evolution on RHEL4, and steve, who provided me the link to the former.

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    Awesome answer. Nearly magical, and very clear and straightforward. Used it with debian VPS without any issues (just had to umount /oldroot/boot of course, on stage 6). I'm linking your answer to other SE questions that had no reponse or negative answer. – vaab Feb 1 '16 at 2:03
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    And resolved, the issue was as @vaab indicated; you must umount /oldroot/boot before you umount /oldroot – ToBeReplaced May 13 '16 at 10:10
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    The point is to unmount and manipulate the root filesystem without needing a physical console. As far as I know, there's no way to keep open a service that reads from a partition while unmounting that partition. If your service doesn't touch the root FS, it's possible you could keep it open using mount --move into the tmpfs, but that is unsupported. – Tom Hunt Oct 7 '16 at 2:01
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    You need to use the OS facilities to restart the init daemon. I've never used upstart, but wiki.ubuntu.com/FoundationsTeam/Specs/… suggests that telinit u might do what you want. – Tom Hunt Jan 31 '17 at 16:09
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    An additional wrinkle I ran into: /tmp is a ramdisk on my system, so I ended up with a ramdisk mounted at /oldroot/tmp, which prevented me from unmounting /oldroot, but doesn't show up in fuser or lsof output. Took a bit of staring at systemd to work that one out... – Chris Kitching Aug 27 '17 at 3:48

If you are sure what you are doing - thus not experimenting, you may hook into initrd which is the non-interactive and fast way.

On a Debian based system here is how.

See the code: https://github.com/szepeviktor/debian-server-tools/blob/master/debian-resizefs.sh

There is another example: https://github.com/szepeviktor/debian-server-tools/blob/master/debian-convert-ext3-ext4.sh

  • When giving an answer it is preferable to give some explanation as to WHY your answer is the one. – Stephen Rauch Feb 15 '17 at 15:33
  • This is a sound approach. I like mine for letting me do the necessary manipulations interactively; however, this one is probably faster. It might be good to edit some more details into the answer itself, or to consider other platforms (it seems this general approach would still work with dracut or mkinitcpio or any other vaguely modern initramfs generator). – Tom Hunt Feb 15 '17 at 17:39
  • Sorry @stephen-rauch I was just pointing out the idea not the execution. – Szépe Viktor Feb 15 '17 at 20:09

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