[Bug 1941649] Re: switch to zstd by default breaks booting focal LTS kernel

[Dimitri John Ledkov]

partial upgrades are not supported, and during upgrades we generally do
not recreate initrds for old kernels.

Meaning one should have at least .old kernel+initrd pair bootable.

It is more of linux bug maybe that v5.4 does not support zstd compressed
initrd?

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Title:
  switch to zstd by default breaks booting focal LTS kernel

Status in initramfs-tools package in Ubuntu:
  New

Bug description:
  While trying to track down a behavior difference between focal and
  impish, I installed focal's kernel in an impish userspace and
  attempted to boot it. The boot failed, unable to find root. While GRUB
  reported that it had loaded the initramfs, no initramfs messages were
  emitted during boot. I tracked this down to initramfs-tools having
  switched to use zstd compression by default. zstd decompression does
  not appear to be supported by the LTS 5.4 kernel - it appears to have
  been added upstream in v5.9. I imagine this is likely to cause
  problems with partial-upgrade scenarios.

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