A file structure for nonerasable media

A file system structure has been designed that is adapted to the peculiarities of write-once optical disk storage media. Key requirements are that the medium be consistent at all times and that large amounts of storage are not wasted in maintaining the directory information appropriate to a normal read/write file system. The file system is implemented in four layers. The first layer replaces the fixed sector-size characteristic of the medium with separate data and control streams composed of variable-length tagged records. The second layer defines data structures used by the control stream: file headers, a series of unmount records, and a journal of directory updates which are pending. The third layer implements a generalized file system (directories, etc.) containing common features of the four target systems: MS-DOS, Unix, VAX/VMS and Macintosh. The fourth layer is the operating system-specific adapter. The objective is to provide a common on-disk structure for all of these systems, while implementing the equivalent of a fully erasable file system. Efficiency of storage space usage and of file reading and writing time are equally important.<>