Most Telcos and Standard Organizations have chosen Openstack as the NFVi because its design allows for almost unlimited scalability, in theory. Reality is that feature increase comes at the cost of management overhead, and currently, a typical Openstack deployment with all the components enable cannot scale linearly beyond a few hundred nodes.
It’s by fine-tuning the 1500+ configuration variables of the control plane that we can achieve real scalability of the compute, network and storage plane, ensuring a consistent behaviour under the most demanding situations. This requires a deep expertise in the field and mastery of all the technological pieces, like the operating system, database engines, networking stacks, etc.
The default parameters of a NFVi infrastructure may not be fine-tuned to avoid bottlenecks and growth issues. You can find interesting experiences on pushing Openstack scaling to handle 168.000 instances in this Ubuntu post and in this video from the Vancouver summit on the crazy things Rackspace had to do to achieve a really big scale deployment.
For more information on the techniques to segregate and scale your environment, visit the Openstack operators guidelines on scaling.