OverviewTo save the eMMC and to allow for faster bootup/shutdowns, I wanted to mount the root partition as read-only. I spent many days researching and trying to find out what areas of the filesystem needed to have write permissions. It seemed like I was going in circles. Unionfs, aufs, mount -bind, overlayfs, tmpfs.....so many options and so many modules that didn't exist in the default BBB Debian kernel. I finally got so fed up that I decided to just start hacking away at the fstab.
DetailsIt turns out that the basic read-only filesystem is fairly easy to create, as everything that needs to be configured is contained in /etc/fstab. We need to change the root mount to read-only and add a couple of temporary file systems to handle logs and such. Let's start with an ordinary fstab created by the eMMC flasher script:
# /etc/fstab: static file system information.
# Auto generated by: beaglebone-black-eMMC-flasher.sh
UUID=716e1ca7-bc61-4958-8c0d-665e48102cfd / ext4 ro,noatime,errors=remount-ro 0 1
UUID=0DFE-E81B /boot/uboot auto defaults 0 0
debugfs /sys/kernel/debug debugfs defaults 0 0
We could then use the following sed command to add the "ro" tag to the root mount line and save it to a temporary fstab file:
> sudo sed s:ext4\ \ noatime:ext4\ \ ro,noatime: /etc/fstab_ro
Next we need to append some tmpfs declarations to our temporary fstab file:
> sudo nano /etc/fstab_ro
and put the following lines at the end:
tmpfs /tmp tmpfs nodev,nosuid,size=32M 0 0
tmpfs /srv tmpfs nodev,size=512K 0 0
tmpfs /var/log tmpfs defaults,noatime,size=1M 0 0
tmpfs /var/tmp tmpfs defaults,noatime,size=512K 0 0
tmpfs /var/run tmpfs defaults,noatime,size=512K 0 0
Replace the real /etc/fstab with your temporary file:
> sudo cp /etc/fstab /etc/fstab.orig
> sudo mv /etc/fstab_ro /etc/fstab
Reboot and you're done!
You may have to experiment with different size limits, depending on your logging needs, but the sizes above should get you started.
Alternate MethodSystemd does the actually mounting, by parsing the fstab. The order in which the mounts are created is not guaranteed. If you need to create mounts in a particular order you need to create a systemd .mount unit for each mount:
> sudo nano /etc/systemd/system/my.mount
with content similar to the following:
Remount RWIf you need to modify the filesystem, you can remount it rw, for a while, with the following command:
> sudo mount -o remount,rw /
To return to read-only mode, either reboot or run this command:
> sudo mount -o remount,ro /