Atomic Host and the kernel

[Josh Boyer]

Hi All,

I'm emailing my questions on the topic here as it seems to be the best
Fedora focused place to discuss Atomic Host and kernel interaction.
If that isn't the case, please point me to where you believe that is.

I have two basic questions around the interaction of Atomic Host and
the kernel.  The first is fairly straightforward: is there anything
Atomic Host or the atomic toolset needs that the kernel does not
provide today?  Missing features, bugs that have been hit but not
fixed, etc.  I believe the answer is likely no, given that atomic is
off and running fine and leverages hardlinks but I thought I would
ask.

The second question is a bit more involved.  Atomic provides the nice
ability for rollback across the entire OS tree.  However, that
requires an atomic image to be spun for every instance of that tree.
That, naturally, means that whenever a new Atomic Host instance is
spun it will use whatever kernel happens to be the latest in the
Fedora release it is built from.  This means that one cannot leverage
the nice side effect of being able to update the kernel independently
of userspace.  (Which is also nice from a testing perspective when it
comes to kernels and regressions.)

To my understanding, the only way to provide such testing would be to
create Atomic Host images that only deviate from the official images
in that they provide a new kernel.  Then one could use the standard
atomic tools to do testing and rollback of _only_ the kernel if a
problem is detected.  While this is certainly possible, I'm not sure
it is something the Cloud sig (or whomever) is really interested in
doing.  On the kernel side, we could provide such images built on our
own but I'm not sure the effort or duplication of
tooling/infrastructure is worthwhile overall.  Particularly when
non-atomic Rawhide continues to be flexible enough for these purposes.

With a two week image release timeframe though, being able to use
different kernels might be a good idea.  Does anyone have any thoughts
around this topic and how to possibly accomplish such testing?  The
only other idea I had was to spin the Atomic Host images containing
the last 3 kernels in them, but I am not sure if choosing between them
at boot is currently possible with multiple kernels installed.

Thanks in advance.

josh