README

This is a proof-of-concept for generating an initramfs that you can use to migrate a commodity Linux to a more reliable embedded setup (such as Nerves).

This repo is built for a specific circumstance and needs adaptation to work in anything else:

• Raspberry Pi OS
• 1 boot partition at mmcblk0p1
• 1 root partition at mmcblk0p2 that needs to be shrunk
• you like the size 3G (otherwise change the init file)
• support for tryboot, should be on most current rpi4s, otherwise shouldn't do any damage

It is easy to put Nerves on a device on your desk. If you have hundred of devices in awkward places in the realy world and you access them via a shell, then changing the file system is a big risk. The big point of this repo is to make it reasonably safe and reliable to migrate a device that is out in the field and needs space on the disk to make room for something like Nerves. So the device will restart twice during the procedure. During that there is a crucial moment of changing the disk. But generally you can add the files whenever you feel like it. You can pull the trigger at your leisure. It requires a special command so the files will be inert unless you decide to commit.

Further requirements

Requires a kernel file packaged up as nerves-kernel8.img, it needs to support initramfs block devices. So if you grab nerves_system_rpi4 and at the time of writing you need main because CONFIG_BLK_DEV_INITRD=y was just added.

I am including a compiled kernel that should work for convenience. You need to copy all files from boot-firmware-files to the /boot/firmware folder on your device.

• tryboot.txt read instead of config.txt if you restart your device with reboot "0 tryboot"
• nerves-kernel8.img kernel image that includes necessary drivers for the initramfs. Chosen by tryboot.txt.
• rootfs.cpio.gz the compiled version of this initramfs. Chosen by tryboot.txt.
• cmdline2.txt uses the initramfs as the root filesystem. Chosen by tryboot.txt.
• config.txt slightly modified, optional, just adds better logging.

The procedure

Using the tryboot mechanism by running sudo reboot "0 tryboot" you tell your RPi to reboot and attempt one (1) tryboot run. It will pick tryboot.txt over config.txt and determine the rest from there. We choose to load an initramfs meaning that we put all of our Linux stuff into memory and can then run operations on the disk that cannot be done while mounted, such as shrinking a partition. So we do. The script contents are in the file configs/rpi4/init.

When they've run the disk should be resized down to that hardcoded value of around 3GB. It then restarts back into RPi OS or whatever you were running. Hopefully no-one being the wiser.

Confirm disk size with df -h.