数据网格中数据复制的研究
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摘要
网格计算是一个广域范围的分布式计算环境,它包括地理位置不同区域上的个体或协会这些协作者之间大规模的资源共享,这些协作者通常被称为虚拟组组织。数据网格是一种以传送和管理海量科学数据并且用于科学分析为为特定需求的网格基础设施。处理大量数据的科学应用和数据网格技术潜在受益的例子包括高能物理、天文学、生物信息学以及地球科学等。
     本文中,我们首先对与复制技术有关的研究成果进行了总结。特别是集中于已经提出的用于数据网格环境的数据副本放置策略。对于每一个副本放置技术,我们考虑它的方法、目标和结果。这些策略通过有关底层的网格拓扑结构,用户的请求模式,数据集的大小以及数据的分布、节点的存储能力等方面的假设有所不同。其他特点包括副本被放置到网格节点的数据请求路径和数据请求方式。面对树结构和别的体系结构之间变化多样的特征,找到一个共同点用于比较不同的复制策略是非常困难的。因此,我们把拓扑结构分为树结构和混合/P2P体系结构,并且分析在每一种体系结构下副本替换策略的影响。一个混合的拓扑结构能够具有树结构和P2P结构的特征,并且能够被用来获取一个复制策略的较好性能
     网格环境中数据复制的主要目标是通过把数据副本放置在接近用户的位置来加强数据的可用性,这样就可以最小化用户的感知响应时间。对于分等级的数据网格,副本通常以自顶向下或自底向上的方式来放置。我们提出了一种两路副本放置模式,它可以把最常用的文件副本放置到距离请求客户端近的位置,把不常用的文件副本分层放置到数据网格根节点下面。由兄弟结点和父结点来为数据请求提供服务。
     另外一个有趣的、和数据网格中文件副本放置有关的问题是副本服务器之间的负载共享。目前大多数的技术都是选择候选结点用于副本放置,这些候选结点具有最大的文件访问请求。但是,在访问负载和存储负载的基础上选择候选结点有可能产生更加有效的负载平衡复制策略,于是,我们提出了一种方法,称为公平-共享复制(FSR),它在分级的数据网格中放置任何副本之前需要考虑数据请求的数量和候选结点的存储负载。
     本文所提出的技术是通过使用GridNet系统来模拟实现的,GridNet系统的研制是为了用来评估数据网格中的复制策略。通过高能物理实验中有关数据网格环境的不同参数的设置来测试两路策略和公平-共享复制策略,性能结果说明了他们的有效性。数据网格环境的不同的设置是指用户访问模式,数据集大小,和服务器存储能力约束。
Grid computing is a wide-area distributed computing environment that involves large-scale resource sharing among collaborations, often referred to as Virtual Organizations, of individuals or institutes located in geographically dispersed areas. Data grids are grid infrastructure with specific needs to transfer and manage massive amounts of scientific data for analysis purposes. Examples of the scientific applications dealing with huge amounts of data and the potential beneficiaries of Data Grid technology are high energy physics, astronomy, bioinformatics, and earth sciences.
     In this thesis we first present a review of current and past research on replication techniques. Specifically, we focus on data replica placement policies proposed for use in the data grid environment. For each replica placement technique, we consider its methodology, objective and results. These strategies differ by the assumptions made regarding underlying grid topology, user request patterns, dataset sizes and their distribution, and storage node capacities. Other distinguishing features include data request path and the manner in which replicas are placed on the Grid nodes. In the presence of diverse and varying characteristics of tree and other architectures it is difficult to create a common ground for juxtaposing different replication strategies. We, therefore, classify the topologies into tree and hybrid/P2P architectures and analyze the impact of replica placement policies in each one. A hybrid topology can carry features of both tree and P2P architectures and thus can be used for better performance of a replication strategy.
     In multi-tier Data Grid, there is a single source of data and it is not feasible for the one server to fulfill the requests of all the users in the Data Grid. Therefore data must be replicated to the other selected nodes in order to reduce the burden on the master server. Replication also facilitates load balancing and improves reliability by creating multiple data copies. Transferring a file from a server to client consumes a huge amount of bandwidth and incurs storage cost. One possible way to reduce the access latency and bandwidth consumption is to replicate data across different sites. However, the files in Grid are big in size i.e., in the magnitude of 500MB-1GB so replication to every site is not feasible. Among many of the challenges, one and also the focus of this thesis, is to find the candidate sites where we can host the replicas. One way to tackle this problem is to place replicas at sites that satisfy the large number of requests. Another approach is to place replica at sites that optimize the transfer time. The data-intensive tasks in scientific applications usually take longer time and therefore considering storage capacity of sites and current storage load is also importance. We can manage Grid storage resources effectively if we place a replica of a file on a site that has less storage load than its neighbors and if its request for the file is above average. In the thesis, both storage status and file requests are considered before placing a replica to a site. Our approach is dynamic so it adapts to change in user behavior and system dynamics.
     The main objective of replication in Grid environment is to enhance data availability by placing replicas at the proximity of users so that user perceived response time is minimized. For a hierarchical Data Grid, replicas are usually placed in either top-down or bottom-up way. We put forward Two-way replica placement scheme that places replicas of most popular files close to the requesting clients and less popular files a tier below from the Data Grid root. We facilitate data requests to be serviced by the sibling nodes as well as by the parent.
     Another interesting issue related to file replica placement in Data Grid is load sharing among replica servers. Most of the current techniques select candidate nodes for replica placement that have maximum access requests for files. However, selecting candidate nodes based on access load and storage load together may result in more effective load balancing replication strategy. We proposed an approach called Fair-share Replication (FSR) that takes into account both the number of requests and the storage load on the candidate sites before placing any replica in hierarchical Data Grid.
     The simulations of proposed techniques were carried out using the GridNet that is developed for evaluating the replication strategies in Data Grid. The Two-way strategy and the Fair-share replication were tested using parameters from High Energy Physics experiments and the performance results demonstrate their effectiveness for the diverse setup of Data Grid environment in terms of user access patterns, dataset sizes, and server storage capacity constraint.
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