overlay root file system with a ssd disk

I think Raspiberry Pi 4 is a great low cost self-hosted solution for a lot of applications such as NAS, Photo Management or gitlab, if it is able to use a ssd disk as its fast and reliable storage.

Since we cann't not boot from ssd card for now, I managed to mount an directory from a ssd storage on top of the root file system just before the standard init process.

What do you need

A raspberry pi 4
a tf-sd card loaded with working raspbian image
A ssd disk with a formated partition labled as "portable" (or you need to change the coresponding name in the overlayRoot.sh)

How to enable it

copy overlayRoot.sh to /sbin/overlayRoot.sh
append "init=/sbin/overlayRoot.sh" to /boot/cmdline.txt
lable your ssd partition "portable".
connect the ssd disk to the rapsberry pi
reboot and wait

pi@raspberrypi:~ $ ls /sbin/overlayRoot.sh
/sbin/overlayRoot.sh

pi@raspberrypi:~ $ cat /boot/cmdline.txt
console=serial0,115200 console=tty1 root=PARTUUID=738a4d67-02 rootfstype=ext4 elevator=deadline fsck.repair=yes rootwait init=/sbin/overlayRoot.sh

# assuming your ssd partition is /dev/sda1
pi@raspberrypi:~ $ sudo e2label /dev/sda1 portable

Effects

If everything goes well, run lsblk and you will get something like this

pi@raspberrypi:~ $ lsblk
NAME        MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sda           8:0    0 953.9G  0 disk              
└─sda1        8:1    0 953.9G  0 part /rw           #This is the partition labled as "portable"
mmcblk0     179:0    0  14.9G  0 disk 
├─mmcblk0p1 179:1    0   256M  0 part /boot         #This is the boot partition of the tf-sd card
└─mmcblk0p2 179:2    0  14.6G  0 part /ro           #This is the original rootfs of the tf-sd card which is read only now

From now on, all writes will be direct to you ssd storage (specifically to the /overlays directory).

How to disable it

remove "init=/sbin/overlayRoot.sh" from /boot/cmdline.txt
reboot

Reference

Inspired by an elegant overlayRoot.sh solution from Pascal Suter for running a raspberry pi from a read-only rootfs mount.

This is a beautiful little script that facilitates running a raspberrypi with a read-only mounted filesystem whereby logfiles and other minor writes would then be written to memory only. The result of this is a stable running system that should be pretty much immune from SD card corruption. Read more on Pascal's site for details.

The original can be found here:

http://wiki.psuter.ch/doku.php?id=solve_raspbian_sd_card_corruption_issues_with_read-only_mounted_root_partition