In this technical report, we analyze NOS file structure for Cyber 170-875 and Cyber 960-31 computer system. We also describe functions, procedure and how-to-operate and how-to-use of VDS. VDS is used to manage large files effectively for Cyber computer system. The purpose of the VDS installation is to increase the virtual disk storage by utilizing magnetic tape, to assist the users of the computer system in managing their files, and to enhance the performance of KAERI Cyber computer system. (Author).