1. A method of allocating a storage volume to a host computer in a system including a plurality of storage systems able to communicate with a plurality of storage virtualizers via a first network, said storage virtualizers being able to communicate with said host computer via a second network, and a management computer able to communicate with said storage systems and said storage virtualizers via a third network, the method comprising: collecting, by the management computer, storage system information on the storage systems in the system and array groups available for each storage system, including available storage space in each array group; collecting by the management computer virtualizer information on the virtualizers in the system; receiving at the management computer an instruction for a storage volume of a specified size to be allocated to the host computer; selecting, by the management computer, a first array group out of said array groups on said storage systems for which said information was collected; requesting that a logical volume be allocated for the specified size in said first array group selected by the management computer; sending a request by the management computer to each virtualizer in the system to determine a virtualizer out of said virtualizers having a lightest current workload; and requesting that the virtualizer having the lightest current workload create a virtual volume mapping to said logical volume and accessible by said host computer.