Raspberry PI read only filesystem with writable overlay

install.sh

#!/bin/bash

cp overlay /etc/initramfs-tools/scripts/
echo "overlay" >> /etc/initramfs-tools/modules
mkdir /overlay /overlay/temp /overlay/base
update-initramfs -c -k `uname -r`
echo "initramfs initrd.img-`uname -r`" >> /boot/config.txt
echo "boot=overlay `cat /boot/cmdline.txt`" > /boot/cmdline.txt

overlay

# Overlay filesystem mounting			-*- shell-script -*-
# 
# HOW TO INSTALL
#
# 1. put this file at /etc/initramfs-tools/scripts/overlay
# 2. run following to add overlay module to initramfs
#     echo "overlay" >> /etc/initramfs-tools/modules
# 3. create overlay directories
#     mkdir /overlay /overlay/temp /overlay/base
# 4. create initramfs
#     update-initramfs -c -k `uname -r`
# 5. modify /boot/config.txt
#     echo "initramfs initrd.img-`uname -r`" >> /boot/config.txt
# 6. add "boot=overlay" to /boot/cmdline.txt
# 7. reboot

mountroot()
{
	local_top
	local_device_setup "${ROOT}" root

	# Get the root filesystem type if not set
	if [ -z "${ROOTFSTYPE}" ]; then
		FSTYPE=$(get_fstype "${ROOT}")
	else
		FSTYPE=${ROOTFSTYPE}
	fi

	local_premount

	ROOT=$(resolve_device "$ROOT")

	# FIXME This has no error checking
	modprobe ${FSTYPE}
	modprobe overlay

	checkfs ${ROOT} root

	mkdir /upper /lower
	mount -t tmpfs tmpfs /upper
	mkdir /upper/data /upper/work

	if [ "${FSTYPE}" != "unknown" ]; then
		mount -r -t ${FSTYPE} ${ROOTFLAGS} ${ROOT} /lower
	else
		mount -r ${ROOTFLAGS} ${ROOT} /lower
	fi

	mount -t overlay -olowerdir=/lower,upperdir=/upper/data,workdir=/upper/work overlay ${rootmnt}
}

mount_bottom()
{
	if [ -d ${rootmnt}/overlay/temp ]; then
		mount -n -o move /upper ${rootmnt}/overlay/temp
	fi
	if [ -d ${rootmnt}/overlay/base ]; then
		mount -n -o move /lower ${rootmnt}/overlay/base
	fi
}