Open Stack

On Thu, 3 Jan 2019 18:02:16 -0600, Matt Riedemann <mriedemos at gmail.com> 
wrote:
> On 1/3/2019 5:45 PM, Dan Smith wrote:
>> You can't abort a post-copy migration once it has started. If we were to
>> add an "always do post-copy" mode to Nova, per the recommendation from
>> the post I linked, then we would start a migration in post-copy mode,
>> which would make it un-cancel-able. That means not only could you not
>> cancel it, but we would have to refuse to start the migration if the
>> user requested an abort action via this new proposed API with any
>> timeout value.
>>
>> Anyway, my point here is just that libvirt already (but not nova/libvirt
>> yet) has a live migration mode where we would not be able to honor a
>> request of "abort after N seconds". If config specified that, we could
>> warn or fail on startup, but via the API all we'd be able to do is
>> refuse to start the migration. I'm just trying to highlight that
>> baking "force/abort after N seconds" into our API is not only just
>> libvirt-specific at the moment, but even libvirt-pre-copy specific.
> 
> OK, sorry, I'm following you now. I didn't make the connection that you
> were talking about something we could do in the future (in nova) to
> initiate the live migration in post-copy mode. Yeah I agree in that case
> if the user said abort we'd just have to reject it and say you can't do
> that based on how the source host is configured.

This seems like a reasonable way to handle the future case of a live 
migration initiated in post-copy mode.

Overall, I'm in support of the idea of adding finer-grained control over 
live migrations, being that we have multiple operators who've expressed 
the usefulness they'd get from it and it seems like a relatively simple 
change.

It also sounds like we have answers for the concerns about bad UX by 
checking pre-live-migration whether the driver supports the new 
parameters and fail fast in that case. And in the future if we have live 
migrations able to be initiated in post-copy mode, fail fast with 
instance action info similarly.

-melanie