[Openstack-operators] [scientific] Resource reservation requirements (Blazar) - Forum session

Jay Pipes jaypipes at gmail.com
Tue Apr 4 17:21:39 UTC 2017

On 04/03/2017 06:07 PM, Blair Bethwaite wrote:
> Hi Jay,
> On 4 April 2017 at 00:20, Jay Pipes <jaypipes at gmail.com> wrote:
>> However, implementing the above in any useful fashion requires that Blazar
>> be placed *above* Nova and essentially that the cloud operator turns off
>> access to Nova's  POST /servers API call for regular users. Because if not,
>> the information that Blazar acts upon can be simply circumvented by any user
>> at any time.
> That's something of an oversimplification. A reservation system
> outside of Nova could manipulate Nova host-aggregates to "cordon off"
> infrastructure from on-demand access (I believe Blazar already uses
> this approach), and it's not much of a jump to imagine operators being
> able to twiddle the available reserved capacity in a finite cloud so
> that reserved capacity can be offered to the subset of users/projects
> that need (or perhaps have paid for) it.

Sure, I'm following you up until here.

> Such a reservation system would even be able to backfill capacity
> between reservations. At the end of the reservation the system
> cleans-up any remaining instances and preps for the next
> reservation.

By "backfill capacity between reservations", do you mean consume 
resources on the compute hosts that are "reserved" by this paying 
customer at some date in the future? i.e. Spot instances that can be 
killed off as necessary by the reservation system to free resources to 
meet its reservation schedule?

> The are a couple of problems with putting this outside of Nova though.
> The main issue is that pre-emptible/spot type instances can't be
> accommodated within the on-demand cloud capacity.

Correct. The reservation system needs complete control over a subset of 
resource providers to be used for these spot instances. It would be like 
a hotel reservation system being used for a motel where cars could 
simply pull up to a room with a vacant sign outside the door. The 
reservation system would never be able to work on accurate data unless 
some part of the motel's rooms were carved out for reservation system to 
use and cars to not pull up and take.

 >  You could have the
> reservation system implementing this feature, but that would then put
> other scheduling constraints on the cloud in order to be effective
> (e.g., there would need to be automation changing the size of the
> on-demand capacity so that the maximum pre-emptible capacity was
> always available). The other issue (admittedly minor, but still a
> consideration) is that it's another service - personally I'd love to
> see Nova support these advanced use-cases directly.

Welcome to the world of microservices. :)


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