1. A method for switching file server from an old file server to a new file server which are coupled via a backend network and for data synchronization, the old file server and the new file server being coupled with one or more host computers via a frontend network to direct input/output (I/O) requests from the one or more host computers, the method comprising: creating stub files in the new file server, the stub files including stub information indicating location of data in the old file server; switching I/O requests directed to the old file server to I/O requests directed to the new file server; checking whether the new file server has any stub files; and if the new file server has one or more stub files, recalling data corresponding to the one or more stub files from the old file server according to the stub information to replace the one or more stub files.