Instances can now be created in preferred cells. Previously, the cell scheduler would select a destination at random from known cells. Filters and weights can now be defined by administrators to direct builds to a particular cell. The keys scheduler_filter_classes and scheduler_filter_classes are configured under the cells group in nova.conf.
The new weights and filters available:
ram_by_instance_type: Select cells with the most capacity for the instance type being requested.
weight_offset: Modifies the DB to weight a particular cell. Also useful for disabling a cell.
mute_child: Assigns negative weight to cells that have not sent an update to their parent cell.
target_cell: Specifies a scheduler.