1. A file sharing system comprising: a plurality of disk devices; a plurality of child server computers, each of the plurality of child server computers manages one of the disk devices; a parent server computer configured to manage the plurality of child server computers; and a plurality of client computers, wherein the disk devices store a large divided file, the plurality of client computers, the plurality of child server computers, and the parent server computer are connected through a network, the plurality of disk devices, each of the plurality of client computers, and each of the plurality of child server computers are connected through a channel switch so that at least each of the plurality of client computers can directly access the plurality of disk device; and the parent server computer comprising: a child server managing unit configured to create real file information, wherein the created real file information specifies a structure of a real file system in each of the disk devices, a structure of a virtual file system over the real file system, and a stored position of a real file in the virtual file system, and a file management database configured to store created real file information, each of the plurality of client computers comprising: a real file information obtaining unit configured to send a request to the parent server computer through the network so as to obtain real file information corresponding to a target file from the parent server computer upon a file-open operation; a real file information registering unit configured to register the real file information obtained by the real file information obtaining unit in a file management table within the respective one of the plurality of client computers; and an I/O requesting unit comprising the functions of: determining whether the real file information corresponding to the target file is stored or not in the file management table upon the file-open operation; sending a request of input/output processing directly to the corresponding child server computer based on the real file information stored in the file management table in a case the target file is stored in the file management table; sending a request to the parent server computer through the network so as to obtain the real file information corresponding to the target file from the parent server computer in a case the target file is not stored in the file management table; and sending a request of inputting/outputting processing directly to the corresponding child server computer based on the obtained real file information so as to perform input/output processing relating to a file operation accessing directly the disk devices through the channel switch according to block I/O information sent by the child server computer.