How to identify root partition via UUID without initramfs/initrd

Question

Without initramfs/initrd support, the following kernel command line won't work:

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

How can I identify my root partition via UUID without the need for an initramfs/initrd?

I can't use a device name like /dev/sda1 either, because the partition resides on a USB-Stick and needs to work on different machines.

Solution 1

I found the answer burried in another thread:

A UUID identifies a filesystems, whereas a PARTUUID identifies a partition (i.e. remains intact after reformatting). Without initramfs/initrd the kernel only supports PARTUUID.

To find the PARTUUID of the block devices in your machine use

sudo blkid

This will print, for example

/dev/sda1: UUID="XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX" TYPE="ext2" PARTUUID="f3f4g3f4-02"

You can now modify you linux command line as follows:

linux   /bzImage root=PARTUUID=f3f4g3f4-02 ro

This will boot from the partition with PARTUUID f3f4g3f4-02, which in this case is /dev/sda1.

Solution 2

lsblk with various options can show you what disk/partition/uuid are in use

eg

% sudo lsblk -o UUID,PARTUUID,NAME,MOUNTPOINT 
UUID                                   PARTUUID           NAME                MOUNTPOINT
                                                          sda                 
d634adc8-69de-edd8-d491-a79e69aeff78   0008500a-01        |-sda1
195237da-8825-45fb-abf7-a62895bd0967                      | `-md0             /boot
d2cf1bcc-d51d-bf37-9723-3b505172fe5f   0008500a-02        `-sda2              
24bvXN-PVU1-kubI-Zgj5-W82i-3Z07-v80lME                      `-md1             
67fe5039-de46-4629-bd03-ee65a5dd0132                          |-godzilla-root /
ba70f1d1-89f0-4dd9-83a4-8bc9a74a6548                          `-godzilla-swap [SWAP]

So I can see that UUID d634adc8-69de-edd8-d491-a79e69aeff78 corresponds to /dev/sda3 and partition UUID 0008500a-01

Depending on your setup you can then do

root=/dev/sda1

or

root=PARTUUID=0008600a-01

(In my case root is part of an LVM and so can't be mounted this way, but the concept applies)

Comments

[daejk]

Thanks for the answer. This was actually my trouble: Without initramfs, I cannot use UUID, but had to use PARTUUID.

[Stephen Harris]

Or you could use the root=/dev/sda1 type construct

[daejk]

I updated the question; I can't use a device name because I'm booting from a USB stick, and the device name varies across different machines.

[Stephen Harris]

OK, answer updated with partuuid stuff from lsblk as well.

[rosch]

The question is about what the root partition is. Why would /dev/sda1 be the root partition? Sudo blkid lists all kind of mounted stuff, including /dev/loop.. whereas blkid lists a lot less. Also, there is lsblk and findmnt for already mounted filesystems.

[MikeBeaton]

UUID= is a thing. So is LABEL=. They are just not supported directly in the kernel (as your code link shows) but rather in the initramfs support of various distros, as this question and answer (for example) shows: stackoverflow.com/questions/38784697/…