Provided is a computer system including a host computer, a plurality of virtual storage systems coupled to the host computer, a storage system coupled to the plurality of virtual storage systems, and a management computer. The storage system includes storage areas provided as volumes to the host computer. Each of the virtual storage systems provides virtual volumes corresponding to the volumes to the host computer. The host computer disperses a plurality of data I/Os containing data to be written in one of the volumes to the virtual volumes of a plurality of paths reaching the one of the volumes at a determined ratio. The management computer calculates a threshold value of a performance value to be set in the virtual volume of each of the paths to determine whether to generate an alert based on the predetermined ratio. Thus, performance of the computer system is properly managed.