How to debug failed boot after successful reconfigure?

[Vladilen Kozin]

Hi list.

I'm trying to wrap my head around running guix. While question in subj
is general it was prompted by a real failure, so really I am
interested both in "general approach" as well as solving the actual
problem I ran into.

having updated with `guix pull` I run `sudo guix system reconfigure
config.scm` which succeeds. I reboot and the boot fails attempting to
mount the new file-systems I've introduced in that config. This being
guix I reboot into older generation. However, now I am unsure how to
debug what'd happened. E.g. I would like to:
- look at failed dmesg or equivalent log,
- look at files that reconfigure produced in /etc, e.g. /etc/fstab

IIUC when I boot even from an older generation, the failed one will be
marked as "current" (that is the newest reconfigured) but everything
in /etc appears to belong to the older generation - hardly surprising.
Following link shown by `guix system describe` in my case
/var/guix/profiles/system-2-link I don't see e.g. /etc/fstab I'm
interested in. Where are those files?

So, the questions above (maybe others I should've asked?) amount to my
asking, how to debug the failure.

My specific case was adding these files-systems to my os declaration:
(file-system
    (mount-point "/mnt/tempb")
    (device (file-system-label "tempb"))
    (type "ext4")
    #;(flags '(no-atime))
    (options "defaults,noatime,discard,user")
    (create-mount-point? #t))

At boot time I got multiple errors from mount unable to figure the
options supplied. This in itself is puzzling seeing how I did check
/etc/fstab after reconfigure and was able to manually mount as per
that generated /etc/fstab without issue. One note is that I think the
(create-mount-point? #t) had not been honored, but I did create those
mount points manually. Dunno, might be a bug in guix proper. But
that's minor.

Thanks

-- 
Best regards
Vlad Kozin