ostree/gnomeos/README-testing-multiroot.md

Experimenting with multiple roots
---------------------------------

	$ mkdir gnomeos-chroot 
	$ qemu-img create gnomeos.raw 2G
	$ mkfs.ext2 -F gnomeos.raw
	$ mount -o loop gnomeos.raw gnomeos-chroot
	$ debootstrap --arch=amd64 squeeze gnomeos-chroot

<http://wiki.debian.org/QEMU#Setting_up_a_testing.2BAC8-unstable_system>

Follow the steps for making a disk image, downloading the business
card CD, booting it in QEMU and running through the installer.  Note I
used the QCOW format, since it is more efficient.  Here are the steps
I chose:

	$ qemu-img create -f qcow2 debian.qcow 2G
	$ qemu-kvm -hda debian.qcow -cdrom debian-testing-amd64-businesscard.iso -boot d -m 512

Test that the image works after installation too, before you start
modifying things below!  Remember to remove the -cdrom and -boot
options from the installation QEMU command.  It should just look like
this:

$ qemu-kvm -hda debian.qcow -m 512


Modifying the image
-------------------

You now have a disk image in debian.img, and the first partition
should be ext4.

The first thing I did was mount the image, and move the "read only"
parts of the OS to a new directory "r0".

	$ mkdir /mnt/debian
	$ modprobe nbd max_part=8
	$ qemu-nbd --connect=/dev/nbd0 debian.qcow
	$ mount /dev/nbd0p1 /mnt/debian/
	$ cd /mnt/debian
	$ mkdir r0
	$ DIRS="bin dev etc lib lib32 lib64 media mnt opt proc root run sbin selinux srv sys tmp usr"
	$ mv $DIRS r0 
	$ mkdir r0/{boot,var,home}
	$ touch r0/{boot,var,home}/EMPTY

Note that /boot, /home and /var are left shared; we create empty
destination directories that will be mounted over.  Now with it still
mounted, we need to move on to the next part - modifying the initrd.

Then I started hacking on the initrd, making understand how to chroot
to "r0".  I ended up with two patches - one to util-linux, and one to
the "init" script in Debian's initrd.

See:
    0001-switch_root-Add-subroot-option.patch
    0001-Add-support-for-subroot-option.patch

$ git clone --depth=1 git://github.com/karelzak/util-linux.git
$ cd util-linux
$ patch -p1 -i ../0001-switch_root-Add-subroot-option.patch
$ ./autogen.sh; ./configure ; make

Now you have a modified "sys-utils/switch_root" binary.  Let's next
patch the initrd and rebuild it:

$ cd ..

Make a backup:

	$ mkdir initrd
	$ cp /mnt/debian/boot/initrd.img-3.0.0-1-amd64{,.orig}

Unpack, and patch:

	$ zcat /mnt/debian/boot/initrd.img-3.0.0-1-amd64 | (cd initrd; cpio -d -i -v)
	$ (cd initrd && patch -p1 -i ../0001-Add-support-for-subroot-option.patch)

Repack:

	$ (cd initrd; find | cpio -o -H newc) | gzip > /mnt/debian/boot/initrd.img-3.0.0-1-amd64.new
	$ mv /mnt/debian/boot/initrd.img-3.0.0-1-amd64{.new,}

Unmount:

	$ umount /mnt/debian

Running hacktree inside the system
----------------------------------

This means that after booting, every process would be in /r0 -
including any hacktree process.  Assuming objects live in say
/objects, we need some way for hacktree to switch things.  I think
just chroot breakout would work.  This has the advantage the daemon
can continue to use libraries from the active host.

Note there is a self-reference here (as is present in Debian/Fedora
etc.) - the update system would at present be shipped with the system
itself.  Should they be independent?  That has advantages and
disadvantages.  I think we should just try really really hard to avoid
breaking hacktree in updates.