Instead of having a read-only (ro) filesystem like pi-star, or foregoing the concept altogether like WPSD, I decided to use an overlayfs like OpenWRT on your router.
This means we have a safe, untouched root partition, but it behaves as if
it’s fully read-write (rw) because all changes get stored in /overlay.
This gives us a nice capability in that we can, more easily than WPSD or other systems:
- at a glance, know what’s been changed from stock
- ‘factory reset’ with no effort whatsoever (just delete everything in
/overlay/upperand reboot) - limited-factory reset, preserving configs while resetting everything else
All this at no real cost to the user.
It might also simplify OS upgrades, but for now I don’t handle those -
upgrading as normal will put all the OS files on /overlay. At worst,
this costs us 400MB of wasted disk space.
How does it work? The kernel boots to partition 2 as normal, and then loads /sbin/preinit as the init command line parameter.
preinit mounts partition 3 to /overlay, does some housekeeping (like
extending partition 3 to use the rest of the disk), sets up the overlay
mount, and then pivot_roots and calls the real /sbin/init.
I leaned heavily on the OpenWRT source and kernel docs to get this going.
I really like it!