create root subvolume

mount /dev/sda1 /tmp/btrfs
btrfs subvolume set-default 5 /tmp/btrfs
cd /tmp/btrfs
btrfs subvolume create @

add “ rootflags=subvol=@ ” to grub kernel options

if update-grub fail check http://edoceo.com/notabene/grub-probe-error-cannot-find-device-for-root this