(c) 2026 Trevor E. Cordes for MUUG
CC-BY-SA
use at your own risk.  no warranty.
VERY DANGEROUS if you don't do things correctly.


ssh -p 22222 root@localhost
  our demo will be on muug in a vm, for ease of demoing and testing

umount -a
  unmounts everything it can;
  will say lots of fs's are "target is busy", that's ok

df
  will confirm the only on-disk fs that remains is root;
  no other: sd* or vd*

mkdir /z
  setup a temporary mountpoint we'll use for our new root fs

mount -t tmpfs none /z
  make a ram fs on it

mkdir /z/{proc,sys,dev,run,usr,var,tmp,oldroot} /z/usr/share
cp -ax /{bin,etc,mnt,sbin,lib,lib64} /z
cp -ax /usr/{bin,sbin,lib,lib64,libexec} /z/usr/
cp -ax /var/{empty,lib,local,lock,nis,opt,preserve,run,spool,tmp,yp} /z/var/
cp -ax /usr/share/terminfo /z/usr/share
cp -ax /usr/share/crypto-policies /z/usr/share
cp -ax /usr/share/empty.sshd /z/usr/share
  makes copies of critical files and programs you'll need to do the work you want
  once in the minimal env; the extra /usr/share stuff is to make systemctl and
  sshd happy, could just copy all of share if you have the ram

mount --make-rprivate /
pivot_root /z /z/oldroot

bash
for i in dev proc sys run; do mount --move /oldroot/$i /$i; done
  you're now running in the temporary root
  all of your previous root files are in /oldroot

systemctl restart sshd
  need to make sure this critical daemon is fully running off the tmp root

from the box you use to access the pivot-root box, start another ssh to make sure
you can still get in:
<whatever you use to ssh into the box you're pivot-rooting on>
ssh -p 22222 root@localhost
  is what I use to get into the demo vm

fuser -vm /oldroot
  you need to get rid of every entry shown

systemctl | grep running |column -t|colrm 90
  start with the systemd-controlled things

killall -e auditd
fuser -vm /oldroot
  systemctl won't allow it to be killed

killall -r systemd-userwork
killall -r systemd-resolve
killall -r systemd-homed
killall -r systemd-userdbd
killall -r sd-pam
killall agetty
fuser -vm /oldroot

systemctl stop chronyd
systemctl stop systemd-oomd
systemctl stop NetworkManager
systemctl stop dbus-broker
fuser -vm /oldroot

systemctl restart systemd-udevd
systemctl restart systemd-journald
systemctl restart systemd-logind.service
fuser -vm /oldroot

systemctl daemon-reexec
fuser -vm /oldroot
  needed to get rid of the systemd entry

from the box you use to	access the pivot-root box:
ssh -p 22222 root@localhost
  and logout the other ssh session

fuser -vm /oldroot

kill ...
fuser -vm /oldroot
  manually run kill on the remaining systemd entry (a user session ps)

umount /oldroot
  we're done! right??
  but it's lying to you! Modern linux has "namespaces" where you can have
  different views of the same system; so your current view says the
  disk/partitions are unused, but they are still held by ps's in other namespaces;

resize2fs /dev/vda3 4G
  errors; device is busy!

grep -R "vda3" /proc/*/mountinfo
  /proc/1204/mountinfo:56 55 253:3 / /oldroot ro,relatime shared:10 - ext4 /de...
  ...

bash
for x in `grep -R vda3 /proc/*/mountinfo | cut -d/ -f 3,3`; do
echo -n "$x "
readlink /proc/$x/ns/mnt
done
  994 mnt:[4026532426]
  ...
  will tell you all of the namespaces the device is still used in;
  you can enter each ns and run the umount there manually:
    nsenter -t 994 -m -- umount /oldroot
    nsenter -t 1204 -m -- umount /oldroot
  or automate...

bash
for x in `grep -R "vda3" /proc/*/mountinfo | cut -d/ -f 3,3`; do
echo -n	"$x "
nsenter -t $x -m -- umount /oldroot
done
  any ones done more than once can just be ignored




manipulations:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::


shrinking root:::::::::::::::::::::::::::::::::::::

e2fsck -f /dev/vda3
  resize2fs will make you do this anyhow

resize2fs /dev/vda3 4G
  shrinks the fs

record the final "(4k) blocks long" count from resize2fs (1048576)

sfdisk --dump /dev/vda |colrm 90
  record these numbers in case you need to revert

echo 'size=4GiB' | sfdisk -N 3 /dev/vda
  shrinks the partition to 4G

sfdisk --dump /dev/vda |colrm 90
  take sfdisk's size= and calc sfdisksizex512/4096 and check it matches the number
  from resize2fs! if not, use the recorded info to reset the partition back to
  original size and debug

expr 8388608 \* 512 / 4096; echo =; echo 1048576


moving root and making /boot bigger:::::::::::::::::

echo '+1GiB,' | sfdisk --move-data /dev/vda -N 3
  moves root 1GiB ahead

echo 'size=+' | sfdisk -N 2 /dev/vda
  grows the /boot partition to max size to fill up the new space between partitions

e2fsck -f /dev/vda2
resize2fs /dev/vda2
  grows the fs



add an lvm partition you can make a vg out of to make an xfs quota home dir::::::::::::

echo 'size=+, type=linux-lvm' | sfdisk --append /dev/vda
  now you have a big lvm block device you can creat a vg/lv's on; do it after you
  check things and reboot as you don't need this minimal environment any more



checking things out:::::::::::::::::::::::::::::::

mkdir /r; mount /dev/vda3 /r
mkdir /b; mount /dev/vda2 /b

ll /r

ll /b

df -h |grep vda
  /r root should be around 4G (from 6)
  /b should be around 2G (from 1)

fdisk -l /dev/vda


get things back to normal:::::::::::::::::::::::::

umount /r
umount /b
sync; sync; sync; sleep 1;
reboot -f
  safe in this case because no on-disk fs's are mounted;
  and don't want to risk normal systemd reboot which just loves to hang indefinitely

uptime

df -h