vfs

The parameter you have to pass to boot from UUID is PARTUUID. So it should be root=PARTUUID=666c2eee-193d-42db-a490-4c444342bd4e.

The documentation explains why it‘s coming back with unknown-block(0,0):

kernel-parameters.txt:

    root=       [KNL] Root filesystem
            See name_to_dev_t comment in init/do_mounts.c.

init/do_mounts.c:

/*
 *  Convert a name into device number.  We accept the following variants:
 *
 *  1) device number in hexadecimal represents itself
 *  2) /dev/nfs represents Root_NFS (0xff)
 *  3) /dev/<disk_name> represents the device number of disk
 *  4) /dev/<disk_name><decimal> represents the device number
 *         of partition - device number of disk plus the partition number
 *  5) /dev/<disk_name>p<decimal> - same as the above, that form is
 *     used when disk name of partitioned disk ends on a digit.
 *  6) PARTUUID=00112233-4455-6677-8899-AABBCCDDEEFF representing the
 *     unique id of a partition if the partition table provides it.
 *     The UUID may be either an EFI/GPT UUID, or refer to an MSDOS
 *     partition using the format SSSSSSSS-PP, where SSSSSSSS is a zero-
 *     filled hex representation of the 32-bit "NT disk signature", and PP
 *     is a zero-filled hex representation of the 1-based partition number.
 *  7) PARTUUID=<UUID>/PARTNROFF=<int> to select a partition in relation to
 *     a partition with a known unique id.
 *
 *  If name doesn‘t have fall into the categories above, we return (0,0).
 *  block_class is used to check if something is a disk name. If the disk
 *  name contains slashes, the device name has them replaced with
 *  bangs.
 */

The last bit at the end says that if it cant understand the value, it returns (0,0), hence your error.

Just to clarify UUIDs are the only reliable way for the kernel to identify hard drives. There are two types: UUID, which is stored in the filesystem and is not available to the kernel at boot-time, and PARTUUID, which is stored in the partition table and IS available at boot time. So you have to use

root=PARTUUID=SSSSSSSS-PP

as /dev/sd?? can change with devices plugged/unplugged.

Don‘t forget to capitalize the hexadecimal number SSSSSSSS-PP you get from blkid!

The more easy to use

root=LABEL=
root=UUID=

only work with an initramfs that fetches these identifiers.

So, if you use a non-empty initramfs, you can have all three! With an empty initramfs, you only have PARTUUID.

This is only partially correct. A partition UUID is completely different from the filesystem‘s UUID, so PARTUUID=666c2eee-193d-42db-a490-4c444342bd4e does not work. However, I was able to use PARTUUID=SSSSSSSS-02 (where SSSSSSSS is the NT disk signature shown just before the error message). – cjm Oct 6 ‘13 at 5:42

So I guess that the real answer is that the kernel does not support root=UUID, only root=PARTUUID. If you want to use a filesystem UUID, I guess you need an initramfs that can handle mounting filesystems by UUID. – cjm Oct 6 ‘13 at 5:48

@cjm my grub boots quite happily with root=UUID. – terdon♦ Oct 6 ‘13 at 14:30

@terdon, I‘ll bet you have an initramfs or initrd. (It could be linked into your kernel instead of being a separate file.) – cjm Oct 6 ‘13 at 14:59

Mind to explain who is using the boot=-argument then? I just used this line for an Archlinuxarm installation that doesn‘t have initrd, and where I can‘t use boot=LABEL nor boot=UUID. – ineiti Aug 21 ‘14 at 22:48

You‘re right - I corrected the boot to root, sorry! Hope it makes more sense now. – ineiti Aug 21 ‘14 at 22:57

Yes. The kernel will interpret a root= flag - but not in the sense i think you mean. There is no such thing as without initramfs - the root= flag points to an initramfs image which the kernel will extract into its own / with cpio. initrd is extinct - and has been since the 2.6 kernel was introduced. – mikeserv Aug 21 ‘14 at 23:01

My understanding (after one day on the forums of archlinuxarm) is that there is no initrd (or initramfs, but kernel.org/doc/Documentation/kernel-parameters.txt calls it initrd) on Archlinuxarm. On Ubuntu and such I do give a pointer to initrd, but (as to my understanding) not on Archlinuxarm. – ineiti Aug 22 ‘14 at 6:54

Archlinuxarm discussion about NO initrd: archlinuxarm.org/forum/viewtopic.php?f=23&t=6652 – ineiti Aug 22 ‘14 at 7:18