Architectural considerations of the NEe mass data file

During the past decade the use of computers in the on-line environment has steadily increased. In particular, there has been a drastic growth of on-line systems for business-oriented applications. We see a shift of interest in computers from their mere computational power to their data management capability, especially in these applications. User demand for the growth of on-line storage capacity in this environment is said to be much larger than the demand for the growth of computational power. We expect the businessoriented computer system to become an information utility in the future. We have been generally using disks as our on-line storage, but the ever-growing demand for file storage capacity forced us to use more inexpensive magnetic tapes, though they are more difficult to handle in the on-line environment. Now it is quite common to see libraries of several thousand tape reels storing tens of billions of bytes of data at large computer installations. Some government data processing in-· stallations actually possess nearly 100,000 tape reels in their tape libraries. The handling of these tape reels require many operators to mount these reels manually on tape units as well as a large floor space for the tape units and the tape library. Furthermore, the traditional tape handling is by nature prone to various human errors. Several mass storage systems (MSS) were developed to solve these problems of traditional magnetic tapes by automating the tape handling process. 1-4 These systems have not yet spread widely because of their high cost, but they seem to be steadily gaining popularity. We believe that these systems will gradually repiace most traditionai tape iibraries and become essential to large information utilities in the years to come. NEC has been engaged in the product planning and development of an MSS. We have studied the existing MSSs in light of the essential requirements of these systems and derived an MSS architecture which is believed to relax the iimitations of the existing systems. As the name of mass storage systems has been historically applied to disks in NEC, we decided to call our newly-developed mass storage system the NEC mass data file subsystem or the MDF subsystem. The present paper describes the major architec-