Comment # 15 on bug 948771 from Michael Chang

As far as I know, root= is needed for generic initramfs, which don't know where
to look up root partition, created by dracut per default without -H,
--hostonly, or without setting this particular setting in dracut.conf to yes.

hostonly="{yes|no}"
    Host-Only mode: Install only what is needed for booting the local host
instead of a   generic host and generate host-specific configuration. 

But still, no good way to detect that initramfs type because user is free to
specify it via dracut command line. For that reason, I think it's valid to
always specify root= in grub.cfg to tell initrd where to look up root partition
and it's /etc/fstab. 

The point I can't understand is why in host-only mode, that root= parameter is
still concerned and used ? Can't root= be neglected because the host related
settings, the /etc/fstab in particular, has been copied to initramfs so why not
just proceed with it ?

Thanks.