The way OSTree supports multiple operating system versions installed is quite simple; we boot into a chroot.

Boot process details

The "normal" boot process looks like this:

  BIOS
  bootloader
  kernel
  initramfs (dracut)
  mount root filesystem on /sysroot
  move /proc /sys from / to /sysroot
  move /sysroot to /
  exec /sbin/init

The OSTree boot process looks like this:

  BIOS
  bootloader
  kernel
  initramfs (dracut)
  mount root filesystem on /sysroot
  parse ostree=OSNAME/TARGET argument from kernel command line, resolving it to /sysroot/ostree/deploy/OSNAME/TARGET
  move /proc /sys from / to /sysroot/ostree/deploy/OSNAME/TARGET
  bind mount /sysroot/ostree/deploy/OSNAME/TARGET/sysroot to /sysroot/ostree/deploy
  bind mount the configuration store /sysroot/ostree/deploy/OSNAME/TARGET-etc to /sysroot/ostree/deploy/OSNAME/TARGET/etc
  bind mount /sysroot/ostree/deploy/OSNAME/TARGET/home to /home  (and the same for /tmp /root)
  bind mount /sysroot/ostree/deploy/OSNAME/var to /sysroot/ostree/deploy/OSNAME/TARGET/var  (makes /var shared among deployments for OSNAME)
  make a readonly bind mount over /sysroot/ostree/deploy/OSNAME/TARGET
  move /sysroot/ostree/deploy/OSNAME/TARGET to /
  exec /sbin/init

Bind mount layout

/home

You'll note in OSTree we share /home with whatever distribution (if any) is installed in /. This implies that we need to share uid/gid mappings.

/var

We don't share /var; that would create a lot of potential sources of conflict. In practice, all operating systems one boots with OSTree must not outright fail when presented with contents of /var they don't expect. Ideally in fact, all data stored there that's parsed should be both forwards and backwards compatible.

/etc

See: OSTree/EverythingInEtcIsABug

/usr

Everything else (/usr, /lib) is considered part of the operating system, and has a read-only bind mount over it. We might later poke a hole through this for /usr/local (which would point to /ostree/local).