Provide a default fall-through driver for the multi scheduler

[David Scannell]

We are building an Openstack Compute extension that has its own high level manager process, similar to nova-compute or nova-volume: https://github.com/gridcentric/openstack.

We want to use the common nova-scheduler to schedule requests to our manager but unfortunately the multi scheduler is hard-codeded for only compute and volume. Basically we want to have a default fall-through option so that our extension, and potentially other ones, can make use the multi scheduler. In other words, if the scheduling request is not destined for the compute or volume topic, then it will use this default driver (e.g. ChangeScheduler) to handle the request.

Attached is a simple patch that defines a new config option "default_scheduler_driver" that defaults to the ChanceScheduler. This driver is then used for non-compute/non-volume topics.

[Vish Ishaya (vishvananda)]

This seems reasonable, although I was considering removing the multi-scheduler completely once the move to volumes is complete and switching the default to the filter scheduler.

[Vish Ishaya (vishvananda)]

* move from nova-volumes to cinder.

[Sean Dague (sdague)]

Proposed priority: Low

Marking as confirmed based on Vish's comments.

David, please follow the process in http://wiki.openstack.org/HowToContribute#If_you.27re_a_developer.2C_start_here: to submit code to the project via gerrit. Thanks!

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/9022

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/9022
Committed: http://github.com/openstack/nova/commit/a4e0a8594b0473fdda220799ca98b0ed18ce05c4
Submitter: Jenkins
Branch: master

commit a4e0a8594b0473fdda220799ca98b0ed18ce05c4
Author: David Scannell <email address hidden>
Date: Tue Jun 26 15:21:22 2012 -0400

    Adds default fall-through to the multi scheduler.
    Fixes bug 1009681

    This offers a default scheduler driver that will be used by the
    multischeduler for requests that are not destined for the
    compute or volume topic. This is useful for openstack extensions
    that want to use the nova-scheduler to schedule requests to
    compute nodes but provide their own manager and topic.

    Change-Id: I753e87fc8979fd0b62529974f00cf11fa55b3d63