Supporting LZ4 as initramfs compressor

[Steve Langasek]

Hi Dimitri,

One point here:

On Wed, Jun 05, 2019 at 01:15:48PM +0100, Dimitri John Ledkov wrote:
> - lz4 size weight over gzip is marginal (14%) but imho worth the
> improved boot time & initrd creation time

A 14% increase in initramfs size is NOT marginal.  Since there are (and will
always be) various scenarios which require a separate /boot partition, we
have over several cycles been contending with how to ensure a clean upgrade
as kernel+initramfs sizes increase over time and push up against the space
limits of the boot partitions that have historically been created.

If you are making a change that increases the size of initramfses by 14%
across the board, you must also:

 - update the ubuntu-release-upgrader code to take this into account so
   users don't run out of space in the middle of the upgrade
   - possibly in a way that is smart enough to know if a user is already
     configuring initramfs-tools to use a non-default compressor, to avoid
     false-positives
 - check that the minimum size requirements for /boot that are encoded in
   the installers are still adequate, or if they need updating (and if they
   need updating, update this back to 18.04)
 - document this issue in the release notes

Thanks,
-- 
Steve Langasek                   Give me a lever long enough and a Free OS
Debian Developer                   to set it on, and I can move the world.
Ubuntu Developer                                   https://www.debian.org/
slangasek at ubuntu.com                                     vorlon at debian.org
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