Boot Chroot

Boot debian/ubuntu based distributions installed in a chroot.

The problem: You like the flexibility of running various systems inside a chroot, but once in a while you need to boot from them. What do you do ?

Option 1: Move all the files into a partition and boot as usual (ughh).

Option 2: use this:

Run make_chroot_initrd script to create a new chroot-enabled initrd image from the existing one:

#  ./make_chroot_initrd /chroot/trusty/boot/initrd.img-3.13.0-32-generic
making new initrd: /chroot/trusty/boot/initrd.img-3.13.0-32-generic.chroot
extracting
patching
packing
done !

The new image will be exactly the same, except now it can handle a chroot= boot parameter.

With grub2 as bootloader you can add an entry to /boot/grub/grub.cfg:
(or perhaps better /etc/grub.d/40_custom)

menuentry "ubuntu trusty, (linux 3.13.0-32) (chroot)" {
	insmod ext2	                  # or whatever you're using ...
	set root='(hd0,7)'                # partition containing the chroot
	set chroot='/chroot/trusty'       # chroot path
	linux	$chroot/boot/vmlinuz-3.13.0-32-generic root=/dev/sda7 chroot=$chroot rw
	initrd	$chroot/boot/initrd.img-3.13.0-32-generic.chroot
}

(change files/partitions to match yours)

Done !

System-wide install

Once you're happy with it you can make the changes permanent
(until initramfs-tools package gets upgraded).
In the chrooted system:

# cd /usr/share/initramfs-tools
# cp -pdrv .  ../initramfs-tools.orig	# backup
# patch -p1 < path_to/boot_chroot/initrd.patch
# rm *.orig */*.orig
# update-initramfs -u

If initrd.patch does not work, especially for Ubuntu 16.04+, please use path_to/boot_chroot/initrd_1604.patch. From now on regular initrd image will support chroot booting.
No need to use a separate initrd.chroot which may get out of sync with it then.

Notes

Tested so far on Ubuntu (precise, trusty) and Debian (wheezy).

Once the initrd is done loading we're chrooted of course. Original partition is still available under /host if such directory exists.

Same approach should work for any linux distribution. The patch used by make_chroot_initrd will probably fail if the init scripts are too different, but shouldn't be too hard to adapt.

Why not use a loop mounted image instead of a chroot ?
It's a lot more efficient cache & io-wise (unless using ploop)