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 ...
I noticed today that Ubuntu 12.04 will not include aufs. What, if anything, will be available to facilitate the creation of union mounts?
I'm trying to mount a unionfs file system in fstab (as one of the steps in this guide), with the following line: unionfs /usr unionfs nodev,noatime,dirs=/.filesystems/usr/overlay=ro:/usr=rw 0 0 ...