rootdev not found by /sbin/init from initramfs
 

Hi,

I experience a problem where the /sbin/init script from my initramfs doesn't find the root device (/dev/sda1) and hence stops at the rescue prompt.

A little context :
I am working on a vmware virtual machine, slackware version is 14.1 i686 (upgraded to current later but it didn't change anything). The partitioning is 1 virtual disk /dev/sda with /dev/sda1 as a btrfs filesystem mounted as / and /dev/sda2 as swap.

The kernel and initramfs are loaded by grub2, the kernel command line is :
Initrd.gz correctly loads the needed modules, including btrfs, and after that the mounting of the root partition fails. The error reported is :
The root cause being that findfs in the /sbin/init script doesn't find the /dev/sda1 partition by its uuid. The ROOTDEV variable is empty, and then the mount command looks like : mount -t btrfs -o ro /mnt

And indeed, when I am at the rescue prompt, the blkid command only return /dev/sda2 (swap partition), no /dev/sda1. Which could explain why findfs doesn't return it either. And yet, /dev/disk/by-uuid/2f9934fd-f987-4903-b26a-e921532cfd1e exists.

If I boot on the slackware DVD now, blkid correctly returns all the partitions including /dev/sda1.

I tried specifying / partition in /etc/fstab by UUID, device or /dev/disk/by-uuid/... but it didn't change anything.

Now I can boot the system without any problem by specifying the root device by its device path (/dev/sda1) in the kernel commandline parameters (root=/dev/sda1). So I am not stuck at all, but I would like to understand why it's failing with UUID and maybe fix a bug if it is one.

PS: It's complicated to provide full file content because copy/paste doesn't work from the VM.

Any help would be appreciated.

Lilo can't boot / on a BTRFS file system. Usual workaround is to make a small ext4 /boot partition, or use another bootloader.

TIP: write the output on an USB stick from the VM, then read it from the host.

J'espère que ça aide.

Perhaps waiting for udevd/mdev to finish may help. If you look in /boot/initrd-tree/init you will see that you can create a file in the initramfs to have the number of seconds to wait for udevd/mdev to finish work.

You set that value via the "-w <wait_time>" option to mkinitrd. You'd have to use the... ...option to /usr/share/mkinitrd/mkinitrd_command_generator.sh to add it to the generated mkinitrd command line.

That's probably why he's using grub2.

Hmm... I should clean my glasses more often. Thanks for the heads up ;)

I already tried that with -w 10 but it didn't help.

You could have mentioned that. :-)

When you are at the rescue prompt, what does show?

It happens to us all. At least you haven't posted a backwards symlink command on the forum like I have. :doh:

It is a link to /dev/sda1 which is whats is expected.

To illustrate the problem, I took the attached screen capture that include results from various diagnostic commands.

Could this be due to a difference between the findfs in busybox used in the initrd, and the one in util-linux?

Now that I've tried to test what the OP is doing, I can say that grub2 can't boot from btrfs either. Or so grub-install tells me in my virtual machine.

So your advice is correct.

I can confirm that Grub2 is able to boot from btrfs.

This is how my slackware VMs are booting up.

So how did you get it installed? I didn't see a way in my VM.

The main caveat with booting on a btrfs filesystem is to leave enough free space between the MBR and the first partition as described on the archwiki.

https://wiki.archlinux.org/index.php...rtition_offset

Apart from that, just include the btrfs module in the initramfs and you are good to go.