Is this a btrfs RAID root?From the prompt, try running the following:modprobe btrfs
udevadm triggerThen try to mount your root partition (remember to use '-o subvol=blahblahblah' if necessary. Then exit the emergency shell.
I think you're unnecessarilly restricting yourself by a wanting a full disk btrfs, particularly if you want to use UEFI or hibernation.. why do you want to restrict yourself in such a way?One tip I can offer is to disable compression on your /boot subvolume. And disable COW on your /var/log directory.
See here: https://wiki.archlinux.org/index.php/Bt … _subvolumeIf you are going to use a MBR/GPT partition to contain a bootable Btrfs filesystem, remember to leave at least 1MB from the beginning of the disk to leave room for GRUB files.