Linux – What is the New /run Filesystem

Short answer -- there is as far as I know no out of the box working solution for your specific requirements. You will have to adjust each initramfs of each distribution to support your specific needs.

Long answer -- yes it is possible. Nowadays most Linux distributions use an initramfs which will be loaded into memory by the bootloader and then unpacked by the kernel. There it will run /sbin/init which is responsible for setting up the early userspace (running udev, loading modules, starting plymouth, asking for crypto passphrase, setting up the network for network mounts, … you name it). As you can run your own scripts and evaluate custom boot parmaters.

Example for Debian

If you are using Debian (should be the same with Ubuntu) you should be able to place a script in /etc/initramfs-tools/scripts/init-bottom/ which will be executded before init is started. For more information about the script, the different directories and the layout have a look at man initramfs-tools. You will have to adjust rootmnt and add the target directory.

Sample (untested) script which should be installed either as /etc/initramfs-tools/scripts/local-bottom/00-myroot or /usr/share/initramfs-tools/scripts/init-top/00-myroot :

#!/bin/sh -e

PREREQS=""

prereqs() { echo "$PREREQS"; }

case "$1" in
  prereqs)
  prereqs
  exit 0
;;
esac

for opt in $(cat /proc/cmdline); do
  case $opt in
    rootdir=*)
      new_mntdir="${opt#rootdir=}"
      ;;
    esac
done

if [ -n "$new_mntdir" ] ; then
  echo rootmnt="$rootmnt/$new_mntdir" >> /conf/param.conf
fi

The idea is to adjust rootmnt which is used in the initramfs init script to start/execute the real init. As the root device is already mounted in the init-bootom stage you can just adjust/alter the target directory.

To use this script just add a new boot parameter, copy the script, make it executable, regenerate your initramfs and add a boot parameter for your Linux distribution, e.g. rootdir=/Ubuntu_Precise.

An attempt at an online solution, but its not quite there.

The setup (in e.g. /tmp directory, as root):

LOWER=$HOME
mkdir u1 w1 o1 O
mount -t overlay overlay -o lowerdir=$LOWER,upperdir=u1,workdir=w1 o1
mount --bind o1 O

Then you can work in O directory, which is an overlay over $LOWER. When you want to do the snapshot:

mkdir u2 w2 o2
mount -t overlay overlay -o lowerdir=o1,upperdir=u2,workdir=w2 o2

(Note that nested overlays like this won't work on older kernels).

But then I want some way to atomically change the bind mount at O to point to o2 instead of o1. I don't know how to do this other than:

umount O
mount --bind o2 O

(Not atomic; there is a window where O is unmounted).

Ideally, running processes could continue to run without knowing that the underlying filesystem of O had changed from o1 to o2. I don't know if this is possible, or whether changing the underlying filesystem of O like this will disrupt open applications too much. I need to investigate further.

Then, once O has been redirected to o2, we can remount o1 read-only as a precaution, then perform an offline merge using for example rdiffdir or overlayfs-tools.

Finally, we would want some way to atomically remount o2 as lowerdir=$HOME,upperdir=u2,workdir=w2 so that o1, u1 and w1 (all now empty dirs) could be removed. Again, I don't know if this is possible.

Otherwise, we can achieve snapshots by just nesting overlays deeper and deeper and leaving the overlay and upper dirs for each mounted without attempting to merge or cleanup. But there is probably a limit to the number of nested overlays that can be mounted. And at some point, we still need to merge the layers downwards if we want to persist changes.