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

[Chris Siebenmann]

The RAID is on partitions, but there is no LVM involved. The layout was
set up through the 24.04 server installer with custom storage layout,
selecting both disks as boot disks, and then using all of their space as
a single partition for the software RAID. The software RAID itself is
unpartitioned.

/proc/mdstat:
md0 : active raid1 vdb2[1] vda2[0]
      41906176 blocks super 1.2 [2/2] [UU]

/proc/partitions:
 253        0   41943040 vda
 253        1       1024 vda1
 253        2   41939968 vda2
 253       16   41943040 vdb
 253       17       1024 vdb1
 253       18   41939968 vdb2
   9        0   41906176 md0

/proc/self/mounts of the root filesystem:
/dev/md0 / ext4 rw,relatime 0 0

I get this test VM into a state with a post-install grub2 update because
I typically work by going through the server installer once,
snapshotting the result, and then working from the snapshot to test our
post-install actions, which involve updating all packages. This grub-pc
problem has happened before several times and previously I've made it go
away by rebuilding the initial install using the current daily server
ISO. This time I'm reporting a bug.

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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 grub2 package in Ubuntu:
  New

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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