Home / ubuntu / Questions / 1198824
In Process
Frederick Ollinger
Asked: 2019-12-27 21:25:48 +0800 CST

Ubuntu 19.10 Drops Into Grub2 Command Line After update-grub On ZFS Rootfs

  • 772

NOTE: Yes, I know I should have taken a snapshot of bpool. This would have solved this whole problem, but I forgot to do this.

I installed Ubuntu 19.10 as a clean install with ZFS on a whole hard drive.

I a few changes to /etc/default/grub:

GRUB_TIMEOUT_STYLE=menu
GRUB_CMDLINE_LINUX_DEFAULT="quiet acpi=off splash"

I ran:

sudo update-grub
sudo grub-install

Then I rebooted.

At this point, I was dropped into the grub command line. There were no menu entries, and my system is unbootable.

I'd really not like to re-install as I had already customized my system and moved lots of personal data onto it.

I ran an Ubuntu rescue disk to fix this. I was able to mount my rpool and bpool partitions and chroot into them, but update-grub and grub-install did not find my current Ubuntu root and did not populate grub.cfg.

I had read that os-prober was a problem:

https://github.com/zfsonlinux/pkg-zfs/issues/193

sudo os-prober

Yields this:

device-mapper: reload ioctl on osprober-linux-sdb5
failed: Device or resource busy
Command failed.

Also, grub-probe fails. Running:

sudo grub-probe /

Yields:

grub-probe: error: unknown filesystem.

So this might explain why grub is not working. Here's the version: 

grub-probe (GRUB) 2.04-1ubuntu12.1

Any ideas on how to make this system bootable again besides re-installing?

grub2 boot zfs 19.10

1 Answers

    1. Before you chroot,

    you need do this (after you have booted from your rescue disk and got things mounted):

    • mv $target_root/dev $target_root/dev.save && mkdir $target_root/dev
    • mount --bind /dev $target_root/dev (you might need to do the same for your /sys)

    Only THEN you can chroot to $target_root/ and expect things to work. After you exit from chroot, don't forget to umount $target_root/dev && rmdir $target_root/dev && mv $target_root/dev.save $target_root/dev (and do the same for /sys if you used that). Or else you won't be able to boot cause your /dev won't be in place.

    1. But actually

    you must be able to install GRUB without all that chroot stuff. Just check /etc/fstab and learn what your partitions are. Get your root (/) device from there.

    Most likely you'll find UUIDs instead of /dev/sd* device names. No problem!! Get device name from ls -l /dev/disk/by-uuid where you find your UUIDs and devices they are linked to.

    Learn where your /boot/grub is located -- most likely it's on your root, so ls /boot/grub is enough to make sure.

    The standard grub-install command WON'T FIND ANYTHING, you need to supply your target installation (root) device to it:

    grub-install /dev/sda (instead of /dev/sda supply your root device you found from /etc/fstab).

    1. Oh, and you might want to check how to invoke grub-probe to get sensible answer from it:

    grub-probe --target=disk / (or any other --target)

    • 0