[openstack-dev] [nova] Proposal for an Experiment

Chris Friesen chris.friesen at windriver.com
Wed Jul 15 20:36:32 UTC 2015

On 07/15/2015 08:18 AM, Ed Leafe wrote:

> What I'd like to investigate is replacing the current design of having
> the compute nodes communicating with the scheduler via message queues.
> This design is overly complex and has several known scalability
> issues. My thought is to replace this with a Cassandra [1] backend.
> Compute nodes would update their state to Cassandra whenever they
> change, and that data would be read by the scheduler to make its host
> selection. When the scheduler chooses a host, it would post the claim
> to Cassandra wrapped in a lightweight transaction, which would ensure
> that no other scheduler has tried to claim those resources. When the
> host has built the requested VM, it will delete the claim and update
> Cassandra with its current state.
> One main motivation for using Cassandra over the current design is
> that it will enable us to run multiple schedulers without increasing
> the raciness of the system.

It seems to me that the ability to run multiple schedulers comes from the fact 
that you're talking about claiming resources in the data store, and not from 
anything inherent in Cassandra itself.

Why couldn't we just update the existing nova scheduler to claim resources in 
the existing database in order to get the same reduction of raciness? (Thus 
allowing multiple schedulers running in parallel.)


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