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I've got a software LVM RAID5 setup, across 4 physical hard drives:

  • /dev/vg1/lv0 contains /boot
  • /dev/vg1/lv1 contains /

A do-release-upgrade from 14.04 to 16.04 failed due to /boot partition being too small, so the whole new kernel was unable to be written to it. I used lvreduce to shrink /home and lvextend to extend /boot by 200MB, and as near as I can tell everything went as expected.

Now when rebooting, grub-rescue pops up with an "unknown filesystem" error, and the only way to boot to the OS is with super-grub-disk. Output of boot info is here.

Running apt-get install --reinstall grub-pc (or similarly update-grub) gives this output:

Preparing to unpack .../grub-pc_2.02~beta2-9ubuntu1.7_amd64.deb ...
Unpacking grub-pc (2.02~beta2-9ubuntu1.7) over (2.02~beta2-9ubuntu1.7) ...
Processing triggers for man-db (2.6.7.1-1ubuntu1) ...
Setting up grub-pc (2.02~beta2-9ubuntu1.7) ...
Generating grub configuration file ...
Warning: Setting GRUB_TIMEOUT to a non-zero value when GRUB_HIDDEN_TIMEOUT is set is no longer supported.
Found linux image: /boot/vmlinuz-3.13.0-88-generic
Found initrd image: /boot/initrd.img-3.13.0-88-generic
/usr/sbin/grub-probe: error: unknown filesystem.
Found linux image: /boot/vmlinuz-3.13.0-44-generic
Found initrd image: /boot/initrd.img-3.13.0-44-generic
done

Running grub-probe -t device / gives:

/dev/mapper/vg1-lv1

and grub-probe -t fs-uuid / gives:

193bbbea-d4f0-48ea-92a0-e520cd173b3c

but running grub-probe -t device /boot and grub-probe -t fs_uuid /boot gives:

/dev/mapper/vg1-lv0
grub-probe: error: unknown filesystem.

Any thoughts on how to go about troubleshooting this?

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