When destinations of memory devices constituting one virtual volume
increase, a relay device performs data transfer among plural memory
devices such that a structure of the virtual volume can be changed so as
to reduce the destinations of the memory devices as much as possible with
this increase in the destinations as an opportunity for data transfer. In
addition, when memory areas with a relatively small capacity increase
among unused memory areas in which a virtual volume is not constituted,
the relay device performs data transfer among the plural memory devices
such that the number of the memory areas with a small capacity is reduced
as much as possible with this increase in the memory areas as an
opportunity for data transfer.