I'm building a few virtual machines under KVM where the base system will be an image (haven't decided specifically which format yet), and all of the remaining system files will be other partitions mounted from a distributed file system.
For instance, / will be part of the main image, but /home will be mounted from elsewhere. My goal is to keep the base system image as small as possible and thus to have as much of the system not be a part of the root partition.
So my question is: Which directories in Linux must be part of the root file system? For instance, I assume that directories such as /etc and /sbin must, because that's where the utilities are that allow booting the system and mounting other partitions. But /opt can certainly be on a different partition.
In case it matters, the systems I'm building will either be Ubuntu Server 12.04 or perhaps 14.04 if I wait until it comes out before producing and deploying these.