Coda A Highly Available File System for a Distributed Workstation Environment

Coda A Highly Available File System for a Distributed Workstation Environment

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IEEE TRANSACTIONS ON COMPUTERS, VOL. 39, NO. 4, APRIL 1990 447 Coda: A Highly Available File System for a Distributed Workstation Environment MAHADEV SATYANARAYANAN, MEMBER, IEEE, JAMES J. KISTLER, PUNEET KUMAR, MARIA E. OKASAKI, ELLEN H. SIEGEL, AND DAVID C. STEERE Abstmct-Coda is a file system for a large-scale distributed computing environment composed of Unix workstations. It pro- vides resiliency to server and network failures through the use of two distinct but complementary mechanisms. One mechanism, server replication, stores copies of a file at multiple servers. The other mechanism, disconnected operution, is a mode of execu- tion in which a caching site temporarily assumes the role of a replication site. Disconnected operation is particularly use- ful for supporting portable workstations. The design of Coda optimizes for availability and performance, and strives to pro- vide the highest degree of consistency attainable in the light of these objectives. Measurements from a prototype show that the performance cost of providing high availability in Coda is reasonable. Index Terms- Andrew, availability, caching, disconnected op- eration, distributed file system, performance, portable comput- ers, scalability, server replication. I. INTRODUCTION LOCATION-transparent distributed file system based on A the Unix' file system model is a valuable mechanism for collaboration between physically dispersed users. This is par- ticularly true in a distributed workstation environment where the primary activities are education, research, and software development. The Andrew File System (AFS) is a highly successful realization of such a mechanism for a campus-sized user community [9], [15]. Positive experience with AFS has motivated the recent work on extending it nationwide [17]. The importance of a shared Unix file system for a distributed workstation environment is further confirmed by many other efforts in industry and academia [16]. The work described in this paper arose from our extensive experience as implementors and users of AFS over the past five years. On the one hand, we were pleased with the func- tionality, performance, and ease of administration of AFS. At the same time we were concerned with its vulnerability to failures of servers and network components. Such failures can Manuscript received July 6, 1989; revised November 19, 1989. This work was supported by National Science Foundation Contract CCR-8657907, De- fense Advanced Research Projects Agency (Order 4976, Contract F33615- 87-C-1499), the IBM Corporation (Faculty Development Award and Gradu- ate Fkllowship), and Digital Equipment Corporation (Equipment Grant). The views and conclusions in this document are those of the authors and do not represent the official policies of the funding agencies or Carnegie Mellon University. The authors are with the School of Computer Science, Carnegie Mellon
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Coda A Highly Available File System for a Distributed Workstation Environment

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