Spread placement groups

A provider network determines which servers to use to host a group of virtual machines for a customer. A placement manager within the provider network employs a placement technique which prioritizes spreading virtual machines for the customer across different "subzones." A subzone is a logical division of a provider network's hardware and/or software denoting risk boundaries. By placing virtual machines in separate subzones, the customer's virtual machines and thus the task collectively performed by the virtual machines are less impacted by a failure within one of the subzones.