远程异步复制系统的数据多缓冲方法

提出了一种在远程异步复制系统(remote asynchronous replication system,RARS)中的数据多缓冲方法.该方法主要包括基于Windows基本卷[3]的海量缓存模型和基于内存的发送缓冲模型.给出了海量缓存和发送缓冲的定义和实现方法,并对该方法进行了性能测试和分析.实验结果表明,该方法解决了磁盘读写速度与网络数据传输速度不匹配的问题,同时不影响本地数据中心数据备份服务的高可用性.     

基于分布式的远程复制系统研究

针对传统的集中式复制结构存在的单点瓶颈和扩展性差等不足,提出了基于分布式结构的数据复制系统。通过将数据同步分散的复制到从节点,使得从节点能在单位时间内获得主节点的最大备份。通过从节点之间点对点的数据复制,使得在保证主从端数据一致性的同时,充分利用了从节点的资源,分散了主节点的负荷。     

浅谈网络块设备的弹性扩容

近些年各个领域的数据都出现爆炸性的增长,而市面上为用户提供的存储服务也越来越多,但大多数存在存储容量受限,增加容量时价格昂贵,灾备空间有限等问题.文章研究了Linux的一个内核模块Network Block Device(NBD),并利用NBD来实现远程存储设备在本地服务器上的弹性挂载,从而实现本地服务的弹性扩容.     

设备复制系统的日志机制研究

设备复制系统采用数据复制技术实现数据容灾,而日志机制是整个系统的核心和基础。本文在分析借鉴文件系统和数据库系统日志的基础上,提出了一个满足设备复制系统特点和设计要求的日志机制,并重点围绕日志的组织结构、生成方式、访问方式以及溢出、修正等特殊情况的处理方法进行了论述。原型实验表明,该日志机制在设备复制系统得到了较好实现,为数据复制提供了保障。     

改进的块复制不等差错保护喷泉码

在多媒体传输、深空通信等应用中,由于数据的重要性程度不同,需要喷泉码具有不等差错保护的特性,根据数据重要性给予不同程度的保护。对基于块复制的不等差错保护喷泉码的机制进行了理论分析,提出了改进的信息符号选取策略,让度1编码符号全部在重要信息比特中选取,一定数量的度2编码符号分别在重要信息比特和次要信息比特中选取,从局部上提升对重要信息比特的保护。仿真结果表明,相比于现有的基于块复制的不等差错保护喷泉码,改进方法在进一步提高重要信息比特保护的同时没有降低对次要信息比特的保护,且有更好的不等恢复时间性能。     

远程异步数据复制系统的设计与实现

远程数据复制技术是远程容灾系统的核心技术,在保持两地间数据一致性和实现灾难恢复中起到关键作用。该文在研究远程数据复制技术的基础上,设计并实现了基于存储设备级的远程异步数据复制系统。系统不占用主机系统资源,对上层的文件系统和应用程序透明,可以降低上层软件设计的复杂程度。     

全局数据仓库环境中数据复制的研究

阐述了数据仓库的基本概念及其体系结构,研究了全局数据仓库环境中的数据复制问题。     

日新月异的复制设备

随着零售店和需求的内容越来越普及,George Cole调查研究了复制设备市场是如何跟上存储和格式更新。     

动态数据复制在分布式信息系统中的应用

提出一种解决分布式数据库数据复制的方案.根据企业现有的计算机软、硬件条件,针对同种、异种数据库,通过采用诸如基于ODBC的高级复制,利用触发器实现动态更新,以及计算机编程实现等多种复制技术的综合应用,在技术实现上做到了经济、合理、方便、灵活,从而既满足了企业应用,又最大的节约了企业成本.     

异步复制流程的研究与性能改进

数据更新流程是远程复制系统的框架。针对不同的应用需求,系统可以采用不同的复制模式。文中主要讨论异步模式下的数据更新流程。在分析借鉴了已有的异步复制协议基础上,提出了一种改进性能的异步复制流程。原型实验表明,此数据更新流程的设计在保持数据的一致性以及系统简单、可靠性的同时,也降低了复制过程中对网络带宽的需求和I/O操作的数量。     

Performance modeling of distributed and replicated databases

The paper surveys performance models for distributed and replicated database systems. Over the last 20 years (1980-2000), a variety of such performance models have been developed and they differ in: (1) which aspects of a real system are or are not captured in the model (e.g., replication, communication, nonuniform data access, etc.); and (2) how these aspects are modeled. We classify the different alternatives and modeling assumptions and discuss their interdependencies and expressiveness for the representation of distributed databases. This leads to a set of building blocks for analytical performance models. To illustrate the work that is surveyed, we select a combination of these proven modeling concepts and give an example of how to compose a balanced analytical model of a replicated database. We use this example to show how to derive meaningful performance values and to discuss the applicability and expressiveness of performance models for distributed and replicated databases. Finally, we compare the analytical results to measurements in a distributed database system     

Storage efficient and secure replicated distributed databases

Data availability and security are two important issues in a distributed database system. Existing schemes achieve high availability at the expense of higher storage cost and data security at the expense of higher processing cost. We develop an integrated methodology that combines the features of some existing schemes dealing with data fragmentation, data encoding, partial replication, and quorum consensus concepts to achieve storage efficient, highly available, and secure distributed database systems     

Synchronization of replicated data in distributed systems

Replication is the key factor in improving the availability of data in distributed systems. Replicated data is stored redundantly at multiple sites so that it can be used by the user even when some of the copies are not available due to site failures. A major restriction in using replication is that replicated copies must behave like a single copy, i.e. mutual consistency as well as internal consistency must be preserved.Synchronization techniques based on the maintenance of multiple versions of data objects have been studied by many researchers in order to increase the degree of concurrency and to reduce the possibility of transaction rollback. Improved availability and increased degree of concurrency would result if multiversion concurrency control techniques can be used with replication control methods in distributed systems. This paper surveys synchronization methods for multiversion distributed systems with replicated data objects that have appeared in the literature. Different synchronization methods are grouped by the underlying mechanisms they use in ordering the operations, and their advantages and disadvantages are discussed. A theory that is used in analyzing the correctness of synchronization methods is reviewed, and a proving method based on the serializability theory is demonstrated using a simple synchronization scheme.     

On the propagation of updates in distributed replicated systems

We analyse the performance and dependability of protocols which implement ‘sequence consistency’ in distributed replicated systems. Sites send messages to each other from time to time, passing information about the update requests that have been received. The recipient of a message is chosen according to some probability distribution which may depend on the sender. The random variables of main interest are (a) the interval between receiving an update request and being able to execute it on the original site, subject to the consistency requirement, and (b) the time it takes to bring all sites to a consistent state after the arrival of an update.     

Optimal cost design of replicated data in distributed database systems

Replicated data are a common strategy for achieving ultra-availability in fault-tolerant distributed database systems. Replication, however, requires additional resources, for example additional costs in terms of maintenance effort, software complexity, hardware requirements and time needed for testing consistency. Various replication control protocols have been developed to ensure data consistency. In this paper, we develop analytical methods for the quorum-consensus replication protocol that minimizes the total system cost by determining the optimal read quorum, the optimal number of system sites, or the optimal site availability assuming that (1) the cost of an individual site is an increasing function of the availability of the site and (2) the two system unavailable modes (i.e. read and write) can have different penalty costs. Several numerical examples and applications are provided to illustrate the results.     

Improving the flexibility for replicated data management in distributed database systems

This paper presents a novel scheme for implementing the flexible replica control protocol[13] in the distributed database systems. The scheme requires less nodes to be locked to perform the read(write) operations. This not only provides better performance, but also provides the system designer extra flexibility to implement the protocol.     

Analysis of a replicated data base

In this paper we study the performance characteristics of a replicated data base under two different updating policies. In the synchronous case requests for any replications of the data base can be processed only if no copies of data base are being updated due to a previous write request, whereas in the non-synchronous case read requests are allowed to be processed at any time if there is a free data base copy. We formulate a queueing theoretic model of the system assuming a Poisson arrival process for both read and write requests. This model is then solved using the matrix geometric solution method and the relevant performance metrics are derived and analyzed.     

The block distributed memory model

We introduce a computation model for developing and analyzing parallel algorithms on distributed memory machines. The model allows the design of algorithms using a single address space and does not assume any particular interconnection topology. We capture performance by incorporating a cost measure for interprocessor communication induced by remote memory accesses. The cost measure includes parameters reflecting memory latency, communication bandwidth, and spatial locality. Our model allows the initial placement of the input data and pipelined prefetching. We use our model to develop parallel algorithms for various data rearrangement problems, load balancing, sorting, FFT, and matrix multiplication. We show that most of these algorithms achieve optimal or near optimal communication complexity while simultaneously guaranteeing an optimal speed-up in computational complexity. Ongoing experimental work in testing and evaluating these algorithms has thus far shown very promising results     

Java based replicated server objects and their remote method invocations in a distributed environment

A distributed system is said to be fault‐tolerant if it is able to provide important services despite partial failures of the computers or software objects in the system. These systems are needed to support applications such as remote access and control, virtual mobile offices and wide area collaborative systems where there are chances of failures in the network and software objects. Fault‐tolerance is usually achieved by replicating the objects in the system. Traditional distributed applications constructed using Java RMI (remote method invocation) are not fault‐tolerant because of the lack of support of object replication. The objective of the present work is to design a remote method invocation that supports server object replication. To provide a fault‐tolerant service to the remote client objects, server objects are actively replicated. The problems associated with the method invocation in the context of active server object replication are presented and solutions are discussed and implemented. Copyright © 2001 John Wiley & Sons, Ltd.     

无线传感器网络中的一种分布式相对定位算法的分析与改进

针对SDGPSN（Scalable and Distributed GPS free Positioning for Sensor Networks）算法通信开销过大的问题,通过改进其局部坐标系和全局坐标系构建过程,提出了一种新的相对定位算法。仿真实验结果显示,改进后的算法较之原算法具有更小的通信开销,使之更适于节点移动频率较低的无线传感器网络的节点定位应用。