Today i looked for a solution to shrink a root filesystem wihtout rebooting a livecd.

After a while i found something on a mailinglist i want to share with you.

Thanks a lot to Andrew Wood and Tom Hunt for this instructions.

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....
```

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...
```
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.
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.
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.

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.

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

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.

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.

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.
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.

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