openstack: relation between controller & compute nodes

Question

I just started playing with openstack, and many things still don't understand. As I see it, to start a VM instance, we normally execute some commands on the controller e.g.

glance image-create nova boot

But how does the controller know: 1) on which compute node to start the VM 2) how many compute nodes it has

Where does it take this information?

Answer 1

The controller will boot determine the location to launch the instance based on the information provided by nova-scheduler:

http://docs.openstack.org/juno/config-reference/content/section_compute-scheduler.html

As for how many compute nodes are recognized, this is determined when you register a compute node with nova compute on the controller. Here is a reference for how compute is installed and configured for RHEL/CentOS/Fedora:

http://docs.openstack.org/juno/install-guide/install/yum/content/ch_nova.html

Answer 2

I'd suggest to learn the OpenStack software architecture for such questions, for example, look at this page http://docs.openstack.org/openstack-ops/content/example_architecture.html.

Simply speacking, OpenStack saves all the configurations in database which is by default mysql, so Controller knows all the information. A Nova component named nova-scheduler running as a controller service will decide where to place VM among all available hosts.

A good staring point is to deploy multiple nodes env. You will know how OpenStack works in the deployment procedure.