Atomic Host and the kernel

[Jason Brooks]

----- Original Message -----
> From: "Josh Boyer" <jwboyer at fedoraproject.org>
> To: "Fedora Cloud SIG" <cloud at lists.fedoraproject.org>
> Cc: "Colin Walters" <walters at redhat.com>
> Sent: Wednesday, August 26, 2015 2:09:23 PM
> Subject: Atomic Host and the kernel
> 
> 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.

atomic-devel at projectatomic.io is another good place

> 
> 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.

I don't think so -- I haven't heard of any such needs...

> 
> 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.

My first thought is that the kernel in an atomic host should just 
work, dammit! :)

There could be different trees reflecting different levels of
kernel maturity. If it's to be one tree, multiple kernels
shipping together, as you mention below, might be the best option --
there's still a grub menu, for choosing, but I'm not positive if
there'd be a conflict w/ any ostree rollback fu.

Jason

> 
> 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
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