[Bug 2060695] Re: 24.04 grub-pc cannot upgrade on mirrored software RAID root disk

[Stefan Fleischmann]

I am seeing the same problem on non-mirrored root disk, using a simple
LVM volume.

I am currently testing the upgrade from 22.04 to 24.04 using `do-
release-upgrade -p`. For the test I deploy a machine with MAAS
installing Ubuntu 22.04, then trying to upgrade to 24.04. Everything
works except the error from grub-install, since it's trying to install
to the LVM volume instead of the disk where the volume is located.

At this point I can either purge the package grub-pc and then install it
again. A simple `dpkg-reconfigure --force grub-pc` does not work, it
shows all the questions EXCEPT the disk selection dialog. Or I can get
the disk selection dialog back by setting:

```
echo 'grub-pc grub-pc/cloud_style_installation boolean false' | debconf-set-selections
```

I also tested running `dpkg-reconfigure grub-pc` before the upgrade and
selected the correct disk. The upgrade afterward fails anyway with the
same error, probably because cloud_style_installation does not get
reset?

Anyway, any idea why the default selection is the LVM volume instead of
the  disk on which the volume resides? Is it cloud-init or MAAS seeding
cloud-init with the wrong value?

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https://bugs.launchpad.net/bugs/2060695

Title:
  24.04 grub-pc cannot upgrade on mirrored software RAID root disk

Status in subiquity:
  Fix Released
Status in cloud-init package in Ubuntu:
  Triaged
Status in grub2 package in Ubuntu:
  Invalid
Status in subiquity package in Ubuntu:
  Fix Released

Bug description:
  I am testing the 24.04 pre-beta in a libvirt virtual machine with two
  /dev/vd* disks set up as a single mirrored software RAID device,
  /dev/md0, that is used for the root filesystem. Since this is a
  libvirt install, it is using BIOS booting, not UEFI (maybe someday
  libvirt will support snapshots of UEFI based VMs). When I attempt to
  install Ubuntu updates, the grub-pc install fails with:

  grub-pc: Running grub-install ...
  Installing for i386-pc platform.
  grub-install: warning: File system `ext2' doesn't support embedding.
  grub-install: warning: Embedding is not possible.  GRUB can only be installed in this setup by using blocklists.  However, blocklists are UNRELIABLE and their use is discouraged..
  grub-install: error: diskfilter writes are not supported.
    grub-install failure for /dev/md0
  You must correct your GRUB install devices before proceeding:

    DEBIAN_FRONTEND=dialog dpkg --configure grub-pc
    dpkg --configure -a
  dpkg: error processing package grub-pc (--configure):
   installed grub-pc package post-installation script subprocess returned error exit status 1

  'debconf-show' reports (changed) settings as:
  * grub-efi/cloud_style_installation: false
  * grub-pc/install_devices: /dev/disk/by-id/md-name-ubuntu-server:0
  * grub-pc/install_devices_empty: false

  The same mirrored root filesystem configuration works on 22.04 LTS.

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