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My system boots OK with this in my GRUB 2 config:

linux   /bzImage root=/dev/sda2 init=/usr/lib/systemd/systemd ro

But if I replace /dev/sda2 with the corresponding UUID:

linux   /bzImage root=UUID=666c2eee-193d-42db-a490-4c444342bd4e init=/usr/lib/systemd/systemd ro

then it fails during boot:

kernel panic - not syncing: VFS: unable to mount root fs on unknown-block(0,0)

The UUID appears to be correct:

# blkid

/dev/sda1: UUID="97ac3744-39de-4d6d-9a81-e3a3ea08a8bb" TYPE="ext2" 

/dev/sda2: UUID="666c2eee-193d-42db-a490-4c444342bd4e" TYPE="ext4" 

Why doesn't it work? Is it because I'm not using an initramfs?

This is x86_64 Gentoo Linux with kernel 3.10.7. I'm using a MBR partition table on sda and a GUID partition table on sdb.

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.

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 can't understand the value, it returns (0,0), hence your error.

This is a 5 year old thread. But still it is imho not fully answered. There's a little example missing. Here it is:

In this example:

/dev/sda3 = /

/dev/sda2 = swap

...using a GPT partition. With MBR (dos partition) the PARTUUID's are shorter but the procedure is the same...

get the PARTUUID's with blkid:

blkid -s PARTUUID -o value /dev/sda3 # root

77fd7830-faa2-4e99-a48b-337ad9eded28

blkid -s PARTUUID -o value /dev/sda2 # swap

5b63167a-6fd2-4e72-948c-90832372956c

/boot/grub/grub.cfg:

search --no-floppy --part-uuid --set=root 77fd7830-faa2-4e99-a48b-337ad9eded28



menuentry "GNU/Linux, KERNEL 4.12.7-lfs-8.1" {

  linux /boot/vmlinuz-4.12.7-lfs-8.1 root=PARTUUID=77fd7830-faa2-4e99-a48b-337ad9eded28 net.ifnames=0 ipv6.disable=1 ro rootwait rootfstype=ext4

}

/etc/fstab/:

PARTUUID=77fd7830-faa2-4e99-a48b-337ad9eded28 / ext4 noatime,nodiratime,errors=remount-ro 0 1

PARTUUID=5b63167a-6fd2-4e72-948c-90832372956c swap swap pri=1 0 0

This is known to WORK with lfs8.1 (kernel 4.12.7) But i think it should work with most other kernels too (olders and newers...)