Is using a read only root file system a good idea for embedded setup?

Question

I have been tasked with running Linux as an operating system on an embedded device.

The target has an x86 processor and has 8 GB CompactFlash device for storage.

I have managed to use buildroot to create the kernel image and cross compilation tools. I have partitioned the CF device into a small FAT partition where the kernel image resides as well as syslinux boot configuration and an ext3 file system where I have decompressed the root file system generated by buildroot to.

The system boots successfully using syslinux by setting the root directory to the CF ext3 partition where my buildroot file system is located.

My question is centred around the need for robustness in the face of immediate (and frequent) power loss as it is crucial for the device to boot successfully after power outages. I have read that mounting the root file system as read only is a way of ensuring data integrity. Is this a sensible way for me to proceed?

I have also read about the possibility of loading the root file system into RAM to achieve the same thing but as yet do not know how to do so.

Is there a preferred way of achieving this goal and if so what is the best way for me to proceed?

Answer 1

Yes, it is a strong solution, but powerfull!

Making r/o useable

You have to mount some directories in rw, like /var, /etc and maybe /home. This could by done using aufs or unionfs. I like this another way, using /dev/shm and mount --bind:

cp -a /var /dev/shm/
mount --bind /dev/shm/var /var

You could before, move all directories who have not to change in normal operation in a static-var, than create symlinks in /var:

mkdir /static-var
mkdir /static-var/cache
mkdir /static-var/lib
mv /var/lib/dpkg /static-var/lib/dpkg
ln -s /static-var/lib/dpkg /var/lib/dpkg
mv /var/cache/apt /static-var/cache/apt
ln -s /static-var/cache/apt /var/cache/apt
... # an so on

So when remounting in ro, copying /var in /dev/shm won't take too much space as most files are moved to /static-var and only symlinks are to be copied in ram.

The better way to do this finely is to make a full power-cycle, one day of full work and finely run a command like:

find / -type f -o -type f -mtime -1

So you will see which files needs to be located on read-write partition.

Logging

As in this host no writeable static memory exist, in order to store history and other logs, you have to config a remote syslog server.

echo >/etc/syslog.conf '*.* @mySyslogServer.localdomain'

In this way, if your system break for any reason, everything before is logged.

Upgrading

When running whith some mount --bind in use, for doing such an upgrade while system is in use (whithout the need of running init 1, for reducing down-time), the simplier way is to re-build a clean root, able to do the upgrade:

After remounting '/' in read-write mode:

mount -o remount,rw /

for mpnt in /{,proc,sys,dev{,/pts}};do
    mount --bind $mnpt /$mnt$mpnt;
    done

chroot /mnt

apt-get update && apt-get dist-upgrade

exit

umount /mnt/{dev{/pts,},proc,sys,}

sync
mount -o remount,ro /

And now:

shutdown -r now