Ubuntu upgrade snag, full /boot partition

[Bob Proulx]

I recently upgraded a few Ubuntu systems.  I have been hearing other
people report that their /boot filled up and failed the upgrade but I
had not seen it.  Not until now.  I looked into what was happening and
thought I would send a hint around.

The Ubuntu installer makes it very difficult to use a different
partitioning than what the installer provides.  And what the installer
provides is a 500MB /boot partition.  And that is becoming problematic
because the Ubuntu kernel upgrade uses significant *temporary* space
in the partition.

    Filesystem      Size  Used Avail Use% Mounted on
    /dev/md0        459M  259M  177M  60% /boot

    -rw------- 1 root root   6250186 Oct 17 14:36 System.map-5.15.0-53-generic
    -rw------- 1 root root   4748126 Oct 17 11:19 System.map-5.4.0-132-generic
    -rw-r--r-- 1 root root    261837 Oct 17 14:36 config-5.15.0-53-generic
    -rw-r--r-- 1 root root    237852 Oct 17 11:19 config-5.4.0-132-generic
    drwx------ 3 root root      4096 Dec 31  1969 efi
    drwxr-xr-x 5 root root      1024 Nov 28 17:17 grub
    -rw-r--r-- 1 root root 113780621 Nov 28 17:18 initrd.img-5.15.0-53-generic
    -rw-r--r-- 1 root root 110402334 Nov 28 17:03 initrd.img-5.4.0-132-generic
    drwx------ 2 root root     12288 Mar 10  2019 lost+found
    -rw-r--r-- 1 root root    182800 Feb  6  2022 memtest86+.bin
    -rw-r--r-- 1 root root    184476 Feb  6  2022 memtest86+.elf
    -rw-r--r-- 1 root root    184980 Feb  6  2022 memtest86+_multiboot.bin
    -rw------- 1 root root  11548224 Oct 17 14:41 vmlinuz-5.15.0-53-generic
    -rw------- 1 root root  13668608 Oct 17 11:42 vmlinuz-5.4.0-132-generic

That looks pretty normal.  Nothing extreme.  Just the defaults.  But
that partition ran out of space for me on an upgrade.

Up through Ubuntu 18.04 they used Linux 4.x kernels.  Upon upgrade
from 18.04 to 20.04 one Linux 4.13 kernel is left behind in /boot and
forgotten.  Then 20.04 leaves one Linux 5.4 kernel behind also
forgotten.  22.04 brings in a Linux 5.15 kernel.

The forgotten Linux 4 kernel (not shown, I purged it for space) was
too much.  This caused the do-release-upgrade script that I ran to
fail out.  And afterward the do-release-upgrade script didn't want to
restart.  So of course I just manually worked through the upgrade.
But I can see where that would leave some people stuck.

So here is the hint.  Prior to an upgrade it would be good to purge
off all Linux kernels behind the one that is currently booting.  Since
we know the current kernel boots and it will become the new backup it
is okay to purge all but the currently running kernel.  And then in
the future I will probably purge off the memtest86+ boot environment
too.  It's sometimes useful but only rarely used.  Removing it would
save 545KB which is miniscule but perhaps every byte counts.

Bob