ENHANCING FILE AVAILABILITY IN DISTRIBUTED SYSTEMS (THE SAGUARO FILE SYSTEM).

Abstract

This dissertation describes the design and implementation of the file system component of the Saguaro operating system for computers connected by a local-area network. Systems constructed on such an architecture have the potential advantage of increased file availability due to their inherent redundancy. In Saguaro, this advantage is made available through two mechanisms that support semi-automatic file replication and access: reproduction sets and metafiles. A reproduction set is a collection of files that the system attempts to keep identical on a "best effort" basis, relying on the user to handle unusual situations that may arise. A metafile is a special file that contains symbolic path names of other files; when a metafile is opened, the system selects an available constituent file and opens it instead. These mechanisms are especially appropriate for situations that do not require guaranteed consistency or a large number of copies. Other interesting aspects of the Saguaro file system design are also described. The logical file system forms a single tree, yet any file can be placed in any of the physical file systems. This organization allows the creation of a logical association among files that is quite different from their physical association. In addition, the broken path algorithm is described. This algorithm makes it possible to bypass elements in a path name that are on inaccessible physical file systems. Thus, any accessible file can be made available, regardless of the availability of directories in its path. Details are provided on the implementation of the Saguaro file system. The servers of which the system is composed are described individually and a comprehensive operational example is supplied to illustrate their interation. The underlying data structures of the file system are presented. The virtual roots, which contain information used by the broken path algorithm, are the most novel of these. Finally, an implementation of reproduction sets and metafiles for interconnected networks running Berkeley UNIX is described. This implementation demonstrates the broad applicability of these mechanisms. It also provides insight into the way in which mechanisms to facilitate user controlled replication of files can be inexpensively added to existing file systems. Performance measurements for this implementation are also presented.