I've come across one of those aufs to overlayfs migration headaches. With aufs you could specify multiple RO-layers and updating some files in them was refreshed with remount.
fstab with aufs:
aufs /etc aufs noauto,br:/rw-root/etc=rw:/ro-root/etc=ro:/etc=rr 0 0
Same functionality with overlayfs:
none /.tmp-root/etc overlayfs noauto,upperdir=/ro-root/etc,lowerdir=/etc 0 0 none /etc overlayfs noauto,upperdir=/rw-root/etc,lowerdir=/.tmp-root/etc 0 0
Mounting the system in order is fine and everything works. The problem arises when I need to update something on the ro-root partition.
Running remount on that sees /etc as the already mounted overlayfs and not the original ro-root. (Solved the striked-out problem with
Apparently the problem is with
inode numbers of the files. So editing of a file works fine, but if I copy a new file over the old one on the lower level, the change is not propagated. So this could be a real overlayfs problem.
I'd really like this setup to continue working (restructuring everything is quite a lot of work and testing that I'd rather avoid, because this affects +50 virtualmachines). That being said, I will also accept answers that would accomplish moving readonly-root after initrd-stage safely to go around this problem and if that's not possible then suggestion most minimalistic way of changing/modifying/creating initrd to accomplish this move.
There's another related question, but it's the simpler form of just two layers. Simple overlayfs reload question