The system automatically picks the most appropriate host to run each virtual machine. End users may specify the zone in which the virtual machine will be created. End users do not have control over which host will run the virtual machine instance.

CloudStack administrators can specify that certain hosts should have a preference for particular types of guest instances. For example, an administrator could state that a host should have a preference to run Windows guests. The default host allocator will attempt to place guests of that OS type on such hosts first. If no such host is available, the allocator will place the instance wherever there is sufficient physical capacity.

Both vertical and horizontal allocation is allowed. Vertical allocation consumes all the resources of a given host before allocating any guests on a second host. This reduces power consumption in the cloud. Horizontal allocation places a guest on each host in a round-robin fashion. This may yield better performance to the guests in some cases. CloudStack also allows an element of CPU over-provisioning as configured by the administrator. Over-provisioning allows the administrator to commit more CPU cycles to the allocated guests than are actually available from the hardware.

CloudStack also provides a pluggable interface for adding new allocators. These custom allocators can provide any policy the administrator desires.