/etc is meant to be on the root filesystem. The sane thing to do is to keep it that way.
If you really really need to split
/etc, you might get better results with a union mount. I don't know if Linux supports union mounts where one of the components matches the mount point, though, and it would be difficult to not make the real
/etc on the root filesystem part of the union mount.
If you want to put all of
/etc on a different partition, you can make a minimal
/etc that contains just what is needed (
/etc/fstab, what init needs (
/etc/rc* or whatever your init system uses), minimal
/etc/group, etc.). Early during boot (a lot earlier than
/etc. But that's going to require a lot of work to get right, and it'll result in
/var/common/etc being the whole
/etc, which is probably not what you want. This setup mostly makes sense for some embedded systems that don't use any of the usual init systems and have a stringent contraint on the size of the root filesystem.
If you have a
/var/common that is shared between many servers, you should approach the problem differently. Use a proper configuration management system to deploy shared files to
A file like
/etc/group should not be shared between machines. Use these files only for local users and groups. For entries that are shared across machines, use NIS or LDAP.