A portable lightweight approach to NFS replication

Under normal circumstances, NFS provides transparent access to remote file systems. Nevertheless, a failure on a single file server compromises the operation of all clients, and thus various replication schemes have been devised to increase file system availability. The approach described in this paper is lightweight in the sense that it strives to make no changes to the NFS protocol nor to the standard NFS client and server code. Rather, a thin layer is introduced between the clients and the original server daemons, which intercepts all NFS requests and propagates the updates to the replicas. Replication is hidden under a primary-secondary update policy and an improved automounter. If the primary server fails, the automounters elect a new primary and remount the relevant file systems. Secondary server failures remain unnoticed by the clients. A prototype version is operational and preliminary results under the Andrew benchmark are presented. The gures obtained show that while read overhead is negligible, the performance of updates is at present impaired by the naive synchronous multi-server write operation

A lightweight approach to NFS replication

Under normal circumstances, NFS provides transparent access to remote file systems. Evertheless,a failure on a single file server compromises the operation of all clients, and thus various replication schemes have been devised to increase file system availability.The approach described in this paper is lightweight in the sense that it strives to make no changes to the NFS protocol nor to the standard NFS client and server code. Rather,a thin layer is introduced between the clients and the original server daemons, which intercepts all NFS requests and propagates the updates to the replicas. Replication is hidden under a primary-secondary update policy and an improved automounter. If the primary server fails, the automounters elect a new primary and remount the relevant file systems. Secondary server failures remain unnoticed by the clients. A prototype version is operational and preliminary result sunder the Andrew benchmark are presented.The figures obtained show that while read overhead is negligible, the performance of updates is severely impaired by the naive synchronous multi-server write operation

Revisiting epsilon serializabilty to improve the database state machine

Recently, a large body of research has been exploiting group communication based techniques to improve the dependability and performance of synchronously replicated database systems

Optimistic total order in wide area networks

Total order multicast greatly simplifies the implementa- tion of fault-tolerant services using the replicated state ma- chine approach. The additional latency of total ordering can be masked by taking advantage of spontaneous order- ing observed in LANs: A tentative delivery allows the ap- plication to proceed in parallel with the ordering protocol. The effectiveness of the technique rests on the optimistic as- sumption that a large share of correctly ordered tentative deliveries offsets the cost of undoing the effect of mistakes. This paper proposes a simple technique which enables the usage of optimistic delivery also in WANs with much larger transmission delays where the optimistic assumption does not normally hold. Our proposal exploits local clocks and the stability of network delays to reduce the mistakes in the ordering of tentative deliveries. An experimental evalu- ation of a modified sequencer-based protocol is presented, illustrating the usefulness of the approach in fault-tolerant database management

Semantic reliability on the database state machine

Database replication protocols based on group communication primitives have recently been the subject of a considerable body of research [1, 11, 13, 6, 8, 4]. The reason for this stems from the adequacy of the order and atomicity properties of group communication primitives to implement synchronous replication (i.e., strong consistent) strategies. Unlike database replication schemes based on traditional transactional

Avaliação de um SGBD replicado usando simulação de redes

A replicação de sistemas de gestão de bases de dados (SGBD) é um mecanismo fundamental para a fiabilidade de sistemas de informação. Em sistemas geograficamente distribuídos é ainda útil na recuperação de desas- tres e disponibilidade ubíqua de dados. Uma técnica de replicação recentemente proposta é a Database State Ma- chine (DBSM), que promete aliar fiabilidade a elevado desempenho tirando partido de sistemas de comunicação em grupo. A avaliação do desempenho desta técnica tem no entanto sido efectuada com redes de comunicação demasiado simples ou irrealistas e com uma carga não representativa. Este artigo propõe uma avaliação rigorosa de uma concretização desta técnica de replicação, aliando um modelo de simulação realista de redes de comunicação com uma geração de carga efectuada de acordo com os padrões elaborados pelo Transaction Processing Council (TPC). Os resultados obtidos confirmam o interesse desta técnica em redes locais, mas mostram que o seu desempenho é condicionado pelas características da rede e da carga.FCT no âmbito do projecto ESCADA - POSI-CHS-33792-

Partial replication in the database state machine

This paper investigates the use of partial replication in the Database State Machine approach introduced ear- lier for fully replicated databases. It builds on the or- der and atomicity properties of group communication primitives to achieve strong consistency and proposes two new abstractions: Resilient Atomic Commit and Fast Atomic Broadcast. Even with atomic broadcast, partial replication re- quires a termination protocol such as atomic commit to ensure transaction atomicity. With Resilient Atomic Commit our termination protocol allows the commit of a transaction despite the failure of some of the par- ticipants. Preliminary performance studies suggest that the additional cost of supporting partial replica- tion can be mitigated through the use of Fast Atomic Broadcast

Group-based replication of on-line transaction processing servers

Several techniques for database replication using group communication have recently been proposed, namely, the Database State Machine, Postgres-R, and the NODO protocol. Although all rely on a totally ordered multicast for consistency, they differ substantially on how multicast is used. This results in different performance trade-offs which are hard to compare as each protocol is presented using a different load scenario and evaluation method. In this paper we evaluate the suitability of such protocols for replication of On-Line Transaction Processing (OLTP) applications in clusters of servers and over wide area networks. This is achieved by implementing them using a common infra-structure and by using a standard workload. The results allows us to select the best protocol regarding performance and scalability in a demanding but realistic usage scenario.Projecto STRONGRE (POSI/CHS/41285/2001) financiado pela Fundação para a Ciência e a Tecnologia (FCT)

MobiSnap: managing database snapshots in a mobile environment

This paper presents MobiSnap, a research project that aims to support the development of SQL based applications for mobile environments, providing configurable support for data divergence control and connectivity abstractions. One of the project goals is to assist the migration of legacy SQL based applications into these new operational platforms

Testing the dependability and performance of group communication based database replication protocols

Database replication based on group communication systems has recently been proposed as an efficient and resilient solution for large-scale data management. However, its evaluation has been conducted either on simplistic simulation models, which fail to assess concrete implementations, or on complete system implementations which are costly to test with realistic large-scale scenarios. This paper presents a tool that combines implementations of replication and communication protocols under study with simulated network, database engine, and traffic generator models. Replication components can therefore be subjected to realistic large scale loads in a variety of scenarios, including fault-injection, while at the same time providing global observation and control. The paper shows first how the model is configured and validated to closely reproduce the behavior of a real system, and then how it is applied, allowing us to derive interesting conclusions both on replication and communication protocols and on their implementationsFundação para a Ciência e a Tecnologia (FCT) - Project STRONGREP (POSI/CHS/41285/2001)