A read-only mount doesn't require the backing store to be immutable, nor does it need open fds on the mount to be closed for the backing store to be updated. I'

A read-only mount doesn't require the backing store to be immutable, nor does it need open fds on the mount to be closed for the backing store to be updated. I've shipped embedded devices that had read-only rootfs mounts and the updater didn't need to reboot before (or after) applying updates. Of course, this was on the Linux kernel, which does support replacing open files. EDIT: I feel obligated to say this: pitchforks down, please. I'm not trying to disparage the macOS kernel. As a designer of software systems interested in how OSes are built, I found the choices taken interesting.