Comment 2 for bug 1508766

I'm surprised the root filesystem is read-only at the point where systemd starts. Isn't it remounted rw by the initramfs? (It is in Debian.)

From context on lp:1508697 you're using some sort of "golden image" creation process: install once, delete unique IDs and other transient state, then dd the image onto multiple machines and let the boot process re-populate those unique IDs.

If your image creation process truncated /etc/machine-id to 0 bytes instead of deleting it altogether, then systemd would be able to do tricks with bind-mounts to populate it; that's what Debian Live does. However, I'm not sure how/whether the newly generated ID gets written to the filesystem when it becomes rw.