1. A method for switching file servers from an old file server to a new file server, the old file server managing a file system which includes a plurality of directories and files, the method comprising: creating a stub file of a root directory of the file system in the new file server; switching a direction of requests, targeting to the plurality of directories and files of the file system, from the old file server to the new file server, after creating the stub file of the root directory in the new file server and before creating all of the other stub files of the plurality of directories and files in the file system; receiving a request at the new file server; checking whether the new file server has any stub files for a target of the request; and if the new file server has one or more stub files for the target of the request, recalling data corresponding to the one or more stub files for the target of the request from the old file server according to stub information indicating location of data to replace the one or more stub files with the recalled data.