Comment 13 for bug 1794318

TL;DR: probably mdadm commit from April 2018; see end.
Today's IRC logs of debugging session with reporter:

  https://irclogs.ubuntu.com/2018/09/26/%23ubuntu-devel.html#t06:41

Current understanding of the situation from log analysis:

  08:21 reboot with (4.15.0-29.31-generic), no problem
  08:21 apt-get --no-upgrade -o Acquire::gpgv::Options::=--ignore-time-conflict -y install grub-efi-amd64-signed
  08:22 apt-get --no-upgrade -o Acquire::gpgv::Options::=--ignore-time-conflict -y install shim-signed
  08:23 apt upgrade

  08:30 reboot with (4.15.0-34.37-generic), no problem
  08:35 sudo apt-get install vagrant -y && sudo reboot
  Setting up mdadm (4.1~rc1-3~ubuntu18.04.1) ...
  Setting up reiserfsprogs (1:3.6.27-2) ...
  Setting up lvm2 (2.02.176-4.1ubuntu3) ...
  Setting up xfsprogs (4.9.0+nmu1ubuntu2) ...
  Setting up xfsprogs (4.9.0+nmu1ubuntu2) ...
  update-initramfs: deferring update (trigger activated) [all above]
  Processing triggers for initramfs-tools (0.130ubuntu3.3) ...
  update-initramfs: Generating /boot/initrd.img-4.15.0-34-generic

 ~08:37, reboot to (4.15.0-34.37-generic), fails, no syslog saved
  "IMSM RAID geometry validation failed." (screenshot)
  "platform does not support raid1 with 2 disks" (screenshot)

  08:38, reboot to (4.15.0-29.31-generic) [manually in Grub]
  08:59, reboot to (4.15.0-29.31-generic) [manually in Grub]

So, not the kernel. But something that (most likely) mdadm changed in
the copy of mdadm copied into the regenerated 'initramfs'.

  Probably running "mdadm --assemble --scan" should also produce
  the same message.

Both kernels booted fine prior to the mdadm update fetched via the
'apt-get install vagrant'.

That brings us to mdadm, looking at the master branch, there was a
commit just prior to the calculation code that generates the "IMSM
RAID geometry validation failed" message:

  https://www.spinics.net/lists/raid/msg60747.html
  https://github.com/neilbrown/mdadm/commit/fcc2c9daede11fcc67e4032fd6fa8da198aaa319#diff-d9ce63e82fa010385c80834480624fa4L7736

RAID 1 is mirroring. The second message
  "platform does not support raid1 with 2 disks"
might well suggest something is non-function.

Reporter has indicated they are not available for several hours, so
further investigation will likely have to wait.

 -Paul