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

[Chris Siebenmann]

I didn't change grub-pc/install-devices, and on our 22.04 BIOS MBR +
mirrored software RAID servers (of which we have a lot), it has the same
value (or the same sort of value, naming the md device). A random 22.04
server install is also using 'super 1.2' for its root /dev/md0 device
superblock format, which will have come from the installer since we
don't change or customize that. We have some remaining 20.04 LTS servers
as well with this same mirrored software RAID root and they are also
superblock 1.2 format and the same grub-pc/install_devices setting. I
think it has been this way in Ubuntu server installs for a long time
(for BIOS MBR, UEFI is slightly different in that it also sets grub-
efi/install_devices to the UEFI partitions on the boot disks).

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

Status in subiquity:
  New
Status in grub2 package in Ubuntu:
  Invalid

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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