Cloned from launchpad blueprint https://blueprints.launchpad.net/nova/+spec/ghosts. Description: "Ghosts" are used to keep track of resources held by the hypervisor for a short period of time after destroying an instance. They work around an issue where hypervisors report some instance resource as free when it is not actually yet free due to some time-consuming process such as memory scrubbing. Specification URL (additional information): https://wiki.openstack.org/wiki/Nova/Ghosts