1. A computer system comprising: a plurality of hosts, each having virtualization software to support execution of one or more virtual machines, the hosts including first and second hosts each having virtualization software of a first type that supports thinly provisioned virtual disks and an unmap command, wherein the unmap command is a command for deallocating blocks previously allocated to a thinly provisioned virtual disk, and a third host having virtualization software of a second type that does not support thinly provisioned virtual disks or the unmap command; and a management server for the virtual machines having a resource scheduling software executed therein to recommend live migration of virtual machines between two hosts having virtualization software of the same type and to not recommend live migration of virtual machines between two hosts having virtualization software of different types.