I think the idea is to have multiple nebula controllers, one per rack or so? From 2nd page of The Register article:

"You can run a cloud with a single Nebula One controller, but the system was designed to have multiple controllers for high availability and resiliency, says Kemp. The Cosmos operating system can currently span as many as five controllers in a single OpenStack controller domain and automatically load-balances work across controllers and the five racks of servers attached to them. With those five racks, you can have on the order of 2,500 cores and 5PB of storage, depending on the servers you pick."

http://www.theregister.co.uk/2013/04/02/nebula_one_openstack...