[openstack-dev] [Nova] Does Nova really need an SQL database?

Chris Friesen chris.friesen at windriver.com
Tue Nov 19 19:37:02 UTC 2013

On 11/19/2013 12:35 PM, Clint Byrum wrote:

> Each scheduler process can own a different set of resources. If they
> each grab instance requests in a round-robin fashion, then they will
> fill their resources up in a relatively well balanced way until one
> scheduler's resources are exhausted. At that time it should bow out of
> taking new instances. If it can't fit a request in, it should kick the
> request out for retry on another scheduler.
> In this way, they only need to be in sync in that they need a way to
> agree on who owns which resources. A distributed hash table that gets
> refreshed whenever schedulers come and go would be fine for that.

That has some potential, but at high occupancy you could end up refusing 
to schedule something because no one scheduler has sufficient resources 
even if the cluster as a whole does.

This gets worse once you start factoring in things like heat and 
instance groups that will want to schedule whole sets of resources 
(instances, IP addresses, network links, cinder volumes, etc.) at once 
with constraints on where they can be placed relative to each other.


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