The new mkinitcpio 16 (yes, we bumped from 0.15 to 16) in testing contains a new feature that automatically generates an "initramfs" on shutdown. systemd will pivot to this image on shutdown. All it does is re-run systemd-shutdown in a ramdisk, thus allowing a proper umount of / and proper shutdown of ALL loop and device mapper devices. This feature is enabled automatically if * /run/initramfs exists (will be created on your next boot by tmpfiles), * /run/initramfs/shutdown doesn't exist or is not an executable, * The user hasn't masked mkinitcpio-generate-shutdown-ramfs.service. This method should also be able to completely obsolete the custom shutdown hook in archiso and the mkinitcpio shutdown hook (using any of those will disable the new stuff).