MapPodsToMachines obtains a list of pods and pivots that list into a map where the keys are host names and the values are the list of pods running on that host.

VolumeZonePredicate evaluates if a pod can fit due to the volumes it requests, given that some volumes may have zone scheduling constraints.

NoDiskConflict evaluates if a pod can fit due to the volumes it requests, and those that are already mounted.