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, including a plurality of hard disk drives, providing a plurality of first virtual volumes to the first file server and providing a plurality of second virtual volumes to the second file server, wherein the disk subsystem includes first and second storage pools, allocates, in response to a write request from the client computer for a write operation to the first volumes, necessary physical storage regions thereto from the first storage pool, and allocates, in response to a write request from the client computer for a write operation to the second volumes, necessary physical storage regions thereto from the second storage pool, wherein the first file server and the second file server provide a file name space including first directory trees and second directory trees to the client computer, each of the first directory trees being respectively stored in different ones of the first virtual volumes, each of the second directory trees being respectively stored in different ones of the second virtual volumes, the first directory trees and the second directory trees being managed by the first file server and the second file server, respectively, wherein each of the first file server and the second file server manages mount point management information indicating, for each of the first and second directory trees, a mount point path name where one of the directory trees is mounted in, the file name space and a server ID managing the directory tree, wherein, in response to a request for migrating one of the first directory trees from the first file server to the second file server, the first file server unmounts the first directory tree to be migrated, the second file server mounts the first directory tree to be migrated on a root tree which the second file server manages, and the first and second file server change a mount point path name of the first directory tree mounted on the root tree which the first file server manages and the server ID from the first file server to the second file server.