Bayou is a replicated, weakly consistent storage system designed for a mobile computing environment that includes portable machines with less than ideal network connectivity. To maximize availability, users can read and write any accessible replica. Bayou's design has focused on supporting apphcation-specific mechanisms to detect and resolve the update conflicts that naturally arise in such a system, ensuring that replicas move towards eventual consistency, and defining a protocol by which the resolution of update conflicts stabilizes. It includes novel methods for conflict detection, called dependency checks, and per-write conflict resolution based on client-provided merge procedures. To guarantee eventual consistency, Bayou servers must be able to rollback the effects of previously executed writes and redo them according to a global senalization order. Furthermore, Bayou permits clients to observe the results of all writes received by a server, Including tentative writes whose conflicts have not been ultimately resolved. This paper presents the motivation for and design of these mechanisms and describes the experiences gained with an initial implementation of the system.

Pangaea is a wide-area file system that supports data sharing among a community of widely distributed users. It is built on a symmetrically decentralized infrastructure that consists of commodity computers provided by the end users. Computers act autonomously to serve data to their local users. When possible, they exchange data with nearby peers to improve the system's overall performance, availability, and network economy. This approach is realized by aggressively creating a replica of a file whenever and wherever it is accessed. This paper presents

The ability to access information on demand when mobile will be a critical capability in the 21st century. In this paper, we examine the fundamental forces at work in mobile computing systems and explain how they constrain the problem of mobile information access. From these constraints, we derive the importance of adaptivity as a crucial requirement of mobile clients. We then develop a taxonomy of adaptation strategies, and summarize our research in application-transparent and application-aware adaptation in the Coda and Odyssey systems respectively.

We describe a novel approach to log-based reconciliation called IceCube. It is general and is parameterised by application and object semantics. IceCube considers more flexible orderings and is designed to ease the burden of reconciliation on the application programmers. IceCube captures the static and dynamic reconciliation constraints between all pairs of actions, proposes schedules that satisfy the static constraints, and validates them against the dynamic constraints. Preliminary experience indicates that strong static constraints

and have found that it is complete and satisfactory in all respects, and that any and all revisions required by the final examining committee have been made.

JetFile is a file system designed with multicast as its distribution mechanism. The goal is to support a large number of clients in an environment such as the Internet where hosts are attached to both high and low speed networks, sometimes over long distances. JetFile is designed for reduced reliance on servers by allowing client-to-client updates using scalable reliable multicast. Clients on high speed networks prefetch large numbers of files. On low speed networks such as wireless, special caching policies are used to decrease file access latency. The prototype implementation of JetFile is on the JetStream gigabit local area network which provides hardware support for many multicast addresses. The multicast Internet backbone (Mbone) is the wide area testbed for JetFile. 1 Introduction To achieve scalability in a wide area network environment, the next generation of distributed file systems need a new paradigm of communication. The prevailing mode of communication for current distrib...

Storage-based intrusion detection allows storage systems to transparently watch for suspicious activity. Storage systems are well-positioned to spot several common intruder actions, such as adding backdoors, inserting Trojan horses, and tampering with audit logs. Further, an intrusion detection system (IDS) embedded in a storage device continues to operate even after client systems are compromised. This paper describes a number of specific warning signs visible at the storage interface. It describes and evaluates a storage IDS, embedded in an NFS server, demonstrating both feasibility and efficiency of storage-based intrusion detection. In particular, both the performance overhead and memory required (40 KB for a reasonable set of rules) are minimal. With small extensions, storage IDSs can also be embedded in block-based storage devices.

Disconnected operation is an important technique for providing mobile access to shared data in distributed file systems. However, data inconsistency resulting from partitioned sharing remains a serious concern. This paper presents a new mechanism called isolation-only transaction(IOT) that uses serializability constraints to automatically detect read/write conflicts. The IOT consistency model provides a set of options for automatic and manual conflict resolution. In addition, application specific knowledge can be incorporated to detect and resolve conflicts. To preserve upward Unix compatibility, the IOT mechanism is provided as an optional file system facility and its flexible interfaces allow any existing Unix application to be executed as an IOT. This paper describes high level system design and implementation and concludes with related work and current status. This research was supported by the Air Force Materiel Command (AFMC) and ARPA under contract number F196828-93-C0193. Addit...

The Bayou architecture provides scalability, availability, extensibility, and adaptability features that address database storage needs of world-wide applications. In addition to discussing these features, this paper presents Bayou's mechanisms for permitting the replicas of a database to vary dynamically without global coordination. Key is the use of weak consistency replication among autonomous machines and strict adherence to the tenet that no operation should involve more than two machines. 1. Introduction Providing scalability, availability, extensibility and adaptability are some of the important challenges faced by designers of world-wide applications and the infrastructure upon which these applications are built. Successful applications running over the emerging global information superhighway must be scalable to support a potentially large, widely-distributed user community. The application's data must be available when any one of the many users needs it. The code base of th...

In this paper we describe a technique called operation-based update propagation for efficiently transmitting updates to large files that have been modified on a weakly connected client of a distributed file system. In this technique, modifications are captured above the file-system layer at the client, shipped to a surrogate client that is strongly connected to a server, re-executed at the surrogate, and the resulting files transmitted from the surrogate to the server. If re-execution fails to produce a file identical to the original, the system falls back to shipping the file from the client over the slow network. We have implemented a prototype of this mechanism in the Coda File System on Linux, and demonstrated performance improvements ranging from 40 percents to nearly three orders of magnitude in reduced network traffic and elapsed time. We also found a novel use of forward error correction in this context.