https://bbs.archlinux.org – Even if all your partitions are btrfs subvolumes, you still have to tell the kernel/initramfs which partition to mount as your rootfs. This is typically done with a kernel command line parameter. So if your root is in fact the root of the btrfs filesystem, it would be rootflags=subvolid=0. Though this can also be done by setting a default /. Something like "btrfs subvo (HowTos)