How to boot into root btrfs file system with minimal initramfs without udev hook?

Arch Linux is UEFI booted from USB flash drive with GPT and three partitions: 1. EFI (vfat file system) 2. root (btrfs file system, was converted from ext4) 3. home (btrfs file system, was converted from ext4) The btrfs partitions have no sub-volumes and are on a single disk (the USB flash memory drive). LVM is not in use here. ### Task Trying to create a **minimal initramfs** by removing udev and a lot of other hooks. Optimizing Bootup With mkinitcpio was used as inspiration too.
The effective mkinitcpio hooks are: base , autodetect and modconf . ### Btrfs hook The btrfs hook is not enabled because mkinitcpio hooks documentation lists for btrfs hook: > This hook is not required for using Btrfs on a single device. ### Regression 1. I have tried to remove udev -> boot error 2. I have tried to add the btrfs module -> boot error 3. I have added btrfs hook -> boot error 4. Changed root=PARTUUID= to root=UUID= notation -> boot error 5. adding paramters rootfstype=btrfs -> boot error 6. rootdelay=0 -> boot error 7. rootdelay=10 -> boot error 8. Mounting using /dev/sda2 from emergency shell -> ok ### Error Only after inserting udev or systemd hooks the system will root into the btrfs root partition, otherwise this error appears: ERROR: device 'PARTUUID=c2...c13' not found. Skipping fsck. :: mounting 'PARTUUID=c2...c13' on real root mount: can't find 'PARTUUID=c2...c13' You are now being dropped into an emergency shell. ### Runtime init debug/log output Enabling boot parameters rd.debug and rd.log shows that "premount" calls the resolve_device function and that returns an empty lookup. resolve_device PARTUUID=c2...c13 local major minor dev tag device=PARTUUID=c2...c13 blkid -lt PARTUUID=c2...c13 -o device dev= The last empty dev is causing the device not found error. ### initramfs mount command mount_handler=default_mount_handler ... # Mount root at /new_root "$mount_handler" /new_root source: https://git.archlinux.org/mkinitcpio.git/tree/init default_mount_handler() { msg ":: mounting '$root' on real root" mount ${rootfstype:+-t $rootfstype} -o ${rwopt:-ro}${rootflags:+,$rootflags} "$root" "$1" source: https://git.archlinux.org/mkinitcpio.git/tree/init_functions#n375 ### initramfs mount version [rootfs ]# mount -V mount from util-linux 2.29.2 (libmount 2.29.2: btrfs, assert, debug) ### initramfs contents $ lsinitcpio -a /boot/initramfs-linux-tiny.img ==> Image: /boot/initramfs-linux-tiny.img ==> Created with mkinitcpio 23 ==> Kernel: 4.10.3-1-ARCH ==> Size: 3.53 MiB ==> Compressed with: lz4 -l -> Uncompressed size: 8.32 MiB (.424 ratio) -> Estimated extraction time: 0.028s ==> Included modules: ahci [explicit] hid-generic [explicit] raid6_pq usbcore atkbd [explicit] i8042 [explicit] scsi_mod usbhid [explicit] btrfs [explicit] libahci sd_mod [explicit] xhci-hcd crc32c-intel [explicit] libata serio xhci-pci [explicit] crc32c_generic libcrc32c serio_raw [explicit] xor ehci-hcd libps2 uas [explicit] ehci-pci [explicit] ohci-hcd usb-common hid ohci-pci [explicit] usb-storage ==> Included binaries: blkid busybox dosfsck fsck fsck.vfat kmod mount switch_root The emergency shell its blkid command lists the correct (PART)UUID value. Could the mounting using using (PART)UUID fail because there is no /dev/disk/? ### Question What is necessary to boot into a non-raid non-subvolume single-drive root btrfs partition located on a USB flash drive without udev? --- PS This error might be caused by a RACE condition, UUID/PARTUUID not yet available when initramfs/init executes the mount ... UUID=... command.