[Openstack-operators] [nova] Should we allow passing new user_data during rebuild?

Clint Byrum clint at fewbar.com
Tue Oct 3 20:00:13 UTC 2017

I fully appreciate that there are users of it today, and that it is
a thing that will likely live for years.

Long lived VMs can use all sorts of features to make VMs work more like
precious long lived servers. However, supporting these cases directly
doesn't make OpenStack scalable or simple. Quite the opposite.

It's worth noting that AD and Kerberos were definitely not designed
for clouds that have short lived VMs, so it does not surprise me that
treating VMs as cattle and then putting them in AD would confuse it.

Excerpts from Tim Bell's message of 2017-10-03 18:46:31 +0000:
> We use rebuild when reverting with snapshots. Keeping the same IP and hostname avoids some issues with Active Directory and Kerberos.
> Tim
> -----Original Message-----
> From: Clint Byrum <clint at fewbar.com>
> Date: Tuesday, 3 October 2017 at 19:17
> To: openstack-operators <openstack-operators at lists.openstack.org>
> Subject: Re: [Openstack-operators] [nova] Should we allow passing new    user_data during rebuild?
>     Excerpts from Matt Riedemann's message of 2017-10-03 10:53:44 -0500:
>     > We plan on deprecating personality files from the compute API in a new 
>     > microversion. The spec for that is here:
>     > 
>     > https://review.openstack.org/#/c/509013/
>     > 
>     > Today you can pass new personality files to inject during rebuild, and 
>     > at the PTG we said we'd allow passing new user_data to rebuild as a 
>     > replacement for the personality files.
>     > 
>     > However, if the only reason one would need to pass personality files 
>     > during rebuild is because we don't persist them during the initial 
>     > server create, do we really need to also allow passing user_data for 
>     > rebuild? The initial user_data is stored with the instance during 
>     > create, and re-used during rebuild, so do we need to allow updating it 
>     > during rebuild?
>     > 
>     My personal opinion is that rebuild is an anti-pattern for cloud, and
>     should be frozen and deprecated. It does nothing but complicate Nova
>     and present challenges for scaling.
>     That said, if it must stay as a feature, I don't think updating the
>     user_data should be a priority. At that point, you've basically created an
>     entirely new server, and you can already do that by creating an entirely
>     new server.
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