1. A storage system comprising: a first file server coupled to a client computer; a second file server coupled to the client computer; and a disk subsystem providing a plurality of virtual volumes to the first file server and the second file server, wherein the first file server and the second file server provide a file name space including directories and files for the client computer, by storing the directories and the files to the plurality of virtual volumes, wherein the first file server and the second file server manage mount point management information indicating a relationship between mount path names of the file name space and the plurality of virtual volumes, wherein each of the mount path names indicates a corresponding one of the directories, wherein the first file server provides a part of the directories and a part of the files, which are under one of the mount point path names, which are stored in a certain virtual volume corresponding to the one of the mount point path names, and which are previously provided by the second file server, wherein the disk subsystem provides a storage pool including a physical storage capacity of the disk subsystem, and the storage pool is corresponded to the file name space, and wherein, in response to a first write request to the plurality of virtual volumes, the disk subsystem assigns the physical storage capacity in the storage pool corresponding to the file name space.