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

[Chad Smith]

For the cloud-init element/task of this bug, before we change our
current upstream grub_dpkg debconf set-selections behavior for boot
device detection, we need to consider the following scenarios where the
absence of cloud-init grub_dpkg behavior may be insufficient.

If Ubuntu primary use-cases determine we should have this cc_grub_dpkg
debconf set-selections behavior off in the majority of our supported
install paths, we'll need to make sure we file bugs against tooling we
are aware of (packer) impacted by this change in behavior. We will also
need to update docs on best practices for golden image creation that
would provide a footnote on this behavior and remediation steps for any
downstream deb-based distro that could be impacted by this change.

** Changed in: cloud-init (Ubuntu)
       Status: New => Triaged

** Changed in: cloud-init (Ubuntu)
   Importance: Undecided => Medium

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

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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