overlayroot：在根目录上挂载覆盖文件系统

# overlayroot mounts an overlay filesystem over the root filesystem I use this for my Raspberry Pi, but it should work on any Debian or derivative. The root file system on the sd-card is mounted read-only on /overlay/lower, and / is a read-write copy on write overlay. There are two sets of instructions below: [Raspbian](#raspbian) and [Ubuntu for ARM](#ubuntu-for-arm). ## Raspbian It uses initramfs. Stock Raspbian doesn't use one so step one would be to get initramfs working. Something like: ```bash sudo mkinitramfs -o /boot/init.gz ``` Add to /boot/config.txt ```bash initramfs init.gz ``` Test the initramfs works by rebooting. It should boot as normal. Add the following line to /etc/initramfs-tools/modules ``` overlay ``` Copy the following files - hooks-overlay to /etc/initramfs-tools/hooks/ - init-bottom-overlay to /etc/initramfs-tools/scripts/init-bottom/ install busybox ```bash sudo apt-get install busybox ``` then rerun ```bash sudo mkinitramfs -o /boot/init.gz ``` Now skip down to [all distributions](#all-distributions) to finish the installation. ## Ubuntu for ARM Add the following line to /etc/initramfs-tools/modules ``` overlay ``` Copy the following files - hooks-overlay to /etc/initramfs-tools/hooks/ - init-bottom-overlay to /etc/initramfs-tools/scripts/init-bottom/ install busybox-static ```bash sudo apt-get install busybox-static ``` then run ```bash sudo update-initramfs -k $(uname -r) -u ``` Now continue to [all distributions](#all-distributions) to finish the installation. ## all distributions add to .bashrc ```bash if [ ! -z "${IMCHROOTED}" ]; then PS1="chroot(${IMCHROOTED})\w:# " fi ``` After rebooting, the root filesystem should be an overlay. If it's on tmpfs any changes made will be lost after a reboot. If you want to upgrade packages, for example, run `rootwork`, the prompt should change to ```bash chroot(/overlay/lower)/:# ``` You're now making changes to the sdcard, and changes will be permanent. I use `rootwork` to work on the real root filesystem. I put it in ~/bin and add ~/bin to my path. The /run directory is problematic to umount, so atm `rootwork` --rbind mounts it on the sd-card root file system, /overlay/lower, and it isn't umounted like /boot /proc /sys and /dev are. After you've finished working on the sd-card run `exit`. `rootwork` tries to clean up by umounting all the mounts it mounted and remount /overlay/lower read-only, but often it can't due to an open file or something else causing the filesystem to be busy. It's probably a good idea to reboot now for 2 reasons: - leaving /overlay/lower read-write could cause file corruption on power loss. - to test it still boots ok after the changes you've just made. Whenever the kernel is updated, for Raspbian you need to rerun ```bash sudo mkinitramfs -o /boot/init.gz ``` and for Ubuntu for ARM ```bash sudo update-initramfs -k $(uname -r) -u ``` TODO: see if there's a hook to automatically run `sudo mkinitramfs -o /boot/init.gz` on kernel install There are comments in some of the files you might want to read and that's about it.