cloud: cells, regions, availability zones, or host aggregates.

    Cells and regions, which segregate an entire cloud and result in running separate Compute deployments.

    Availability zones and host aggregates, which merely divide a single Compute deployment.

Cell(A single API endpoint for compute)

The top-level cell ("API cell") has a host that runs the nova-api service, but no nova-compute services.
Each child cell runs all of the other typical nova-* services (except for the nova-api service)

* runs nova-cells

Regions (separate API endpoints and no coordination between regions)

* OpenStack dashboard (horizon) currently uses only a single region, so one dashboard service should be run per region

Availability zone

OpenStack compute hosts into logical groups and provides a form of physical isolation and redundancy from other availability zones

Host aggregates zone

you might use host aggregates to partition an availability zone into groups of hosts that either share common resources, such as storage and network, or have a special property, such as trusted computing hardware.