1. A system, comprising: a plurality of storage systems forming a virtualized storage pool, each of the plurality of storage systems comprising a controller and a plurality of storage devices and each of the plurality of storage systems managing one or more logical volumes provided with the plurality of storage devices, and a management server managing the virtualized storage pool, wherein a first storage system of the plurality of storage systems comprises a first backup logical volume of a primary logical volume at a first timing, wherein a second storage system of the plurality of storage systems comprises the primary logical volume and a second backup logical volume of the primary logical volume at a second timing after the first timing; wherein the first backup logical volume in the first storage system is adapted to be used, at the first timing, for restoring data of the primary logical volume in the second storage system based on generation information of the first backup logical volume and the second backup logical volume, the generation information managed by either the management server or at least one storage system of the plurality of storage systems.