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overlayroot

mounts an overlay filesystem over the root filesystem

I use this for my Raspberry Pi, but it should work on any Debian or derivative.

The root file system on the sd-card is mounted read-only on /overlay/lower, and / is a read-write copy on write overlay.

It uses initramfs. Stock Raspbian doesn't use one so step one would be to get initramfs working. Something like:

sudo mkinitramfs -o /boot/init.gz

Add to /boot/config.txt

initramfs init.gz

Test the initramfs works by rebooting. It should boot as normal.

Add the following line to /etc/initramfs-tools/modules

overlay

Copy the following files

  • hooks-overlay to /etc/initramfs-tools/hooks/
  • init-bottom-overlay to /etc/initramfs-tools/scripts/init-bottom/

install busybox

sudo apt-get install busybox

then rerun

sudo mkinitramfs -o /boot/init.gz

add to .bashrc

if [ ! -z "${IMCHROOTED}" ]; then
        PS1="chroot(${IMCHROOTED})\w:# "
fi

After rebooting, the root filesystem should be an overlay. If it's on tmpfs any changes made will be lost after a reboot. If you want to upgrade packages, for example, run rootwork, the prompt should change to

chroot(/overlay/lower)/:#

You're now making changes to the sdcard, and changes will be permanent.

I use rootwork to work on the real root filesystem. I put it in ~/bin and add ~/bin to my path.

The /run directory is problematic to umount, so atm rootwork --rbind mounts it on the sd-card root file system, /overlay/lower, and it isn't umounted like /boot /proc /sys and /dev are.

After you've finished working on the sd-card run exit. rootwork tries to clean up by umounting all the mounts it mounted and remount /overlay/lower read-only, but often it can't due to an open file or something else causing the filesystem to be busy. It's probably a good idea to reboot now for 2 reasons:

  • leaving /overlay/lower read-write could cause file corruption on power loss.
  • to test it still boots ok after the changes you've just made.

Whenever the kernel is updated you need to rerun

sudo mkinitramfs -o /boot/init.gz

TODO: see if there's a hook to automatically run sudo mkinitramfs -o /boot/init.gz on kernel install

There are comments in some of the files you might want to read and that's about it.