GCaching--一种网格协同缓存系统

协同缓存通过多个代理缓存服务器的协同工作,可以充分利用各服务器的缓存空间,提高缓存命中率．网格技术可以方便地共享和整合异构的服务器资源．结合网格和协同技术,提出了一种网格协同缓存系统Gcaching,它将地域上分布的多个代理缓存服务器组成缓存池,充分利用缓存资源,协同工作,为用户提供更好的服务能力．Gcaching系统设计并实现了一种缓存放置和替换算法GCPR,它使用周期缓存更新策略,根据用户访问模式自适应地调整缓存数据的分布．仿真实验表明,GCPR算法的缓存命中率和平均访问跳数都优于传统的LRU等算法．     

异构分布式数据资源中的网格文件访问

为对网格文件进行数据访问,提出一个带有标准服务器和协议的高性能存储系统——联众系统。采用集群环境中的数据管理方法,通过使用多个标准数据服务器访问多个站点,采用的技术包括Grid FTP以及OGSA Byte I/O界面等。实验结果证明,联众系统能够被用作真实网格环境中的并行文件系统,并获得较好的数据访问结果。     

Linux虚拟文件系统实现技术探讨

文章论述了Linux文件系统的逻辑关系和逻辑结构，对VFS中几个主要的数据结构进行了分析，剖析了Linux文件系统中一个物理文件系统的安装与注册、VFS的产生，以及通过VFS管理和访问物理文件系统的内核工作机制。     

WebGIS中带图业务数据的缓存和预取机制研究

目前在WebGIS中地图缓存已得到深入的关注和研究,随着WebGIS客户端性能的逐渐增强,如图片音频等越来越丰富的内容也能被展示出来;由于服务端磁盘读取和网络传输两大瓶颈,一定程度上延长了用户访问这些资源的等待时间;以某高速公路管理系统为背景系统,采用了在服务端和客户端各加一组业务数据缓存、客户端启动后预先读取服务端缓存的方式,提高了带图片的业务数据的访问性能;同时根据该WebGIS中业务数据特有的地理位置关系,在分析用户访问轨迹后,客户端预测用户可能访问的下一条数据并预取,进一步提高了缓存性能。     

Grid Portal的原理和实现

网格门户(Grid Portal)是为科学领域的用户提供Web方式访问网格计算资源的WWW应用。Grid Portal底层采用Globus Toolkit中间件, 为授权用户访问网格资源提供便利,并提供增强的网格服务功能。采用Grid Portal能够大大拓展网格的应用范围。该文分析了Grid Portal的结 构和运作原理,并介绍了采用Gridport技术开发网格门户的经验。     

Linux虚拟文件系统机制

论述了Linux文件系统的逻辑关系和逻辑结构，对VFS中几个主要的数据结构进行了分析，剖析了Linux文件系统中一个物理文件系统的安装与注册、VFS的产生，以及通过VFS来管理和访问物理文件系统的内核工作机制。对开发及设计出适合不同需求的专用文件系统有着参考意义。     

基于网格的煤矿安全应用网格体系研究

煤矿安全应用工作中数据信息分布、计算资源闲置、异构以及计算功能单一严重制约着行业应用和信息化的发展。网格（grid）是新一代高性能计算环境和信息服务基础设施,能够实现动态跨地域的资源共享和协同工作。该文基于新兴的网格技术,参照OGSA体系,提出并构建了从信息获取、处理到应用的完整的煤矿安全应用网格（CMSAG）体系结构。同时结合行业应用需求,着重研究了其资源表示、资源管理、存储访问以及数据缓存等关键技术,最后通过具体应用示例的工作流程论证了该网格体系的应用模式。     

一种加速广域文件系统读写访问的缓存策略

针对广域网高延迟、低带宽的特性给广域文件系统访问带来的性能影响问题,提出了一种不依赖于底层文件系统、能够加速广域文件系统读写访问的缓存策略.该策略支持基于区间粒度的文件数据缓存及访问,并支持元数据本地缓存;该策略提供基于阈值的容量管理功能,采用超时与最终一致相结合的方式维护缓存的一致性.最后使用典型的文件I/O基准测试工具和元数据性能测试工具对该缓存策略进行了评测,实验结果表明:该缓存策略减少了客户端与服务器的交互次数,给广域文件系统的数据访问带来了明显的性能提升,当缓存命中时其数据的读写性能与本地文件系统相近.     

基于补丁算法的流媒体代理缓存的研究与实现

对流媒体代理缓存和流调度技术进行了分析与研究，结合代理缓存和补丁算法的优点，设计与实现了一种基于补丁算法的流媒体代理缓存的高效方法。分析和实验研究表明，该方法能进一步减少对骨干网络带宽资源和原始服务器的磁盘I/O资源的占用，提高用户访问请求的响应速度。     

基于内存网格的磁盘缓存设计与实现

内存对计算机系统的性能具有重要影响,内存网格能够共享跨域的开放网络环境中的内存资源,以磁盘缓存的形式提高系统性能.为实现缓存对应用的透明性,提出了动态修改操作系统内核的二进制代码.实现文件系统读写流程的截获和重定向;并提出了基于内核线程的异步缓存写入方法.提高写缓存的效率.通过原型系统及实验,说明上述方法既不需要修改鹰用程序、也不需要修改操作系统源代码,并且能充分利用共享的内存资源+提高系统的I/O性能.     

网格cache获取资源副本的优化策略

网格中资源能力的不均衡和异构特性会给系统造成瓶颈,引起客户端延迟,为了提高网格的性能,在网格中引入了cache技术,有效地缓解了这一问题。在cache中没有检索到需要的文件时,网格的SRM如何从网格中以一个最快、最有效的方式来获取一个该对象的副本,并将其置于本地的cache中,是一个迫切需要研究的问题。该文在引入经济学模型的基础上,运用遗传算法,给出了该问题的一个有效解决策略,经过初步分析,该算法可以达到满意的结果。     

倒排文件索引缓存机制的优化

为了有效提高搜索引擎检索服务系统的整体性能,提出了一种基于倒排文件索引的缓存机制优化方法。具体研究过程是：首先分析倒排文件缓存的体系结构和数据加载,接着讨论负载数据对倒排文件缓存和缓存替换算法的影响,最后通过设计仿真实验研究倒排文件的缓存优化。研究结果表明,采用倒排文件索引的缓存机制优化方法可以明显减少磁盘系统I/O访问次数,提高磁盘系统带宽的利用率。     

Design a progressive video caching policy for video proxy servers

Proxy servers have been used to cache web objects to alleviate the load of the web servers and to reduce network congestion on the Internet. In this paper, a central video server is connected to a proxy server via wide area networks (WANs) and the proxy server can reach many clients via local area networks (LANs). We assume a video can be either entirely or partially cached in the proxy to reduce WAN bandwidth consumption. Since the storage space and the sustained disk I/O bandwidth are limited resources in the proxy, how to efficiently utilize these resources to maximize the WAN bandwidth reduction is an important issue. We design a progressive video caching policy in which each video can be cached at several levels corresponding to cached data sizes and required WAN bandwidths. For a video, the proxy server determines to cache a smaller amount of data at a lower level or to gradually accumulate more data to reach a higher level. The proposed progressive caching policy allows the proxy to adjust caching amount for each video based on its resource condition and the user access pattern. We investigate the scenarios in which the access pattern is priorly known or unknown and the effectiveness of the caching policy is evaluated.     

PDDRA: A new pre-fetching based dynamic data replication algorithm in data grids

In recent years, grid technology has had such a fast growth that it has been used in many scientific experiments and research centers. A large number of storage elements and computational resources are combined to generate a grid which gives us shared access to extra computing power. In particular, data grid deals with data intensive applications and provides intensive resources across widely distributed communities. Data replication is an efficient way for distributing replicas among the data grids, making it possible to access similar data in different locations of the data grid. Replication reduces data access time and improves the performance of the system. In this paper, we propose a new dynamic data replication algorithm named PDDRA that optimizes the traditional algorithms. Our proposed algorithm is based on an assumption: members in a VO (Virtual Organization) have similar interests in files. Based on this assumption and also file access history, PDDRA predicts future needs of grid sites and pre-fetches a sequence of files to the requester grid site, so the next time that this site needs a file, it will be locally available. This will considerably reduce access latency, response time and bandwidth consumption. PDDRA consists of three phases: storing file access patterns, requesting a file and performing replication and pre-fetching and replacement. The algorithm was tested using a grid simulator, OptorSim developed by European Data Grid projects. The simulation results show that our proposed algorithm has better performance in comparison with other algorithms in terms of job execution time, effective network usage, total number of replications, hit ratio and percentage of storage filled.     

一种机群文件系统的缓存模型

提出了一种新的机群文件系统缓存模型，它充分利用机群系统累积的系统资源和高速的互联网络，将文件系统元数据和内容数据分离，分别使用分布式元数据缓存和统一缓存模型进行管理。元数据缓存使用改进的广播一致性协议和LRU替换算法。内容数据统一缓存则将磁盘缓存整个文件和内存缓存文件块相结合组成一个单一映像的多层次分布协作缓存，并使用单拷贝优先LRU和向前传递调度缓存块替换算法以及一种贪心的数据预取方法。实验结果表明，这两种缓存机制结合使用能极大地提高机群文件系统的性能。     

Proxy caching for peer-to-peer live streaming

Peer-to-Peer (P2P) live streaming has become increasingly popular over the Internet. To alleviate the inter-ISP traffic load and to minimize the access latency, proxy caching has been widely suggested for P2P applications. In this paper, we carry out an extensive measurement study on the properties of P2P live streaming data requests. Our measurement demonstrates that the P2P living streaming traffic exhibits strong localities that could be explored by caching. This is particularly noticeable for the temporal locality, which is often much weaker in the conventional P2P file sharing applications. Our results further suggest that the request time of the same data piece from different peers exhibits a generalized extreme value distribution. We then propose a novel sliding window (SLW)-based caching algorithm, which predicts and caches popular data pieces according to the measured distribution. Our experimental results suggest that the P2P live streaming can greatly benefit from the proxy caching. And, with much lower overhead, our SLW algorithm works closer to an off-line optimal algorithm that holds the complete knowledge of future requests.     

The Gecko NFS Web proxy

The World-Wide Web provides remote access to pages using its own naming scheme (URLs), transfer protocol (HTTP), and cache algorithms. Not only does using these special-purpose mechanisms have performance implications, but they make it impossible for standard Unix applications to access the Web. Gecko is a system that provides access to the Web via the NFS protocol. URLs are mapped to Unix file names, providing unmodified applications access to Web pages; pages are transferred from the Gecko server to the clients using NFS instead of HTTP, significantly improving performance; and NFS's cache consistency mechanism ensures that all clients have the same version of a page. Applications access pages as they would Unix files. A client-side proxy translates HTTP requests into file accesses, allowing existing Web applications to use Gecko. Experiments performed on our prototype show that Gecko is able to provide this additional functionality at a performance level that exceeds that of HTTP.     

Tulip: A New Hash Based Cooperative Web Caching Architecture

With the exponential growth of WWW traffic, web proxy caching becomes a critical technique for Internet web services. Well-organized proxy caching systems with multiple servers can greatly reduce the user perceived latency and decrease the network bandwidth consumption. Thus, many research papers focused on improving web caching performance with the efficient coordination algorithms among multiple servers. Hash based algorithm is the most widely used server coordination mechanism, however, there's still a lot of technical issues need to be addressed. In this paper, we propose a new hash based web caching architecture, Tulip. Tulip aggregates web objects that are likely to be accessed together into object clusters and uses object clusters as the primary access units. Tulip extends the locality-based algorithm in UCFS to hash based web proxy systems and proposes a simple algorithm to reduce the data grouping overhead. It takes into consideration the access speed dispatch between memory and disk and replaces expensive small disk I/O with less large ones. In case a client request cannot be fulfilled by the server in the memory, the system fetches the whole cluster which contains the required object into memory, the future requests for other objects in the same cluster can be satisfied directly from memory and slow disk I/Os are avoided. It also introduces a simple and efficient data dupllication algorithm, few maintenance work need to be done in case of server join/leave or server failure. Along with the local caching strategy, Tulip achieves better fault tolerance and load balance capability with the minimal cost. Our simulation results show Tulip has better performance than previous approaches.     

Efficient data distribution in a Web server farm

High-performance Web sites rely on Web server `farms', hundreds of computers serving the same content, for scalability, reliability, and low-latency access to Internet content. Deploying these scalable farms typically requires the power of distributed or clustered file systems. Building Web server farms on file systems complements hierarchical proxy caching. Proxy caching replicates Web content throughout the Internet, thereby reducing latency from network delays and off-loading traffic from the primary servers. Web server farms scale resources at a single site, reducing latency from queuing delays. Both technologies are essential when building a high-performance infrastructure for content delivery. The authors present a cache consistency model and locking protocol customized for file systems that are used as scalable infrastructure for Web server farms. The protocol takes advantage of the Web's relaxed consistency semantics to reduce latencies and network overhead. Our hybrid approach preserves strong consistency for concurrent write sharing with time-based consistency and push caching for readers (Web servers). Using simulation, we compare our approach to the Andrew file system and the sequential consistency file system protocols we propose to replace     

Improving energy efficiency for flash memory based embedded applications

The JFFS2 file system for flash memory compresses files before actually writing them into flash memory. Because of this, multimedia files, for instance, which are already compressed in the application level go through an unnecessary and time-consuming compression stage and cause energy waste. Also, when reading such multimedia files, the default use of disk cache results in unnecessary main memory access, hence an energy waste, due to the low cache hit ratio. This paper presents two techniques to reduce the energy consumption of the JFFS2 flash file system for power-aware applications. One is to avoid data compression selectively when writing files, and the other is to bypass the page caching when reading sequential files. The modified file system is implemented on a PDA running Linux and the experiment results show that the proposed mechanism effectively reduces the overall energy consumption when accessing continuous and large files.