多层服务器集群容量规划启发方法研究

本文研究了多层服务器集群系统的容量规划问题,提出以吞吐优化为目标的增量式服务器资源配置算法SHISA.该方法基于闭环排队网络模型求解系统的稳态性能指标,利用请求队列长度、资源利用率及有效响应时间等启发信息指导服务器的增量配置过程.对不同启发信息下算法的求解能力进行了敏感性分析.     

基于并行容错网的对等服务器集群的设计与实现

为了提高服务器系统的可靠性、可用性和性能,并改变目前非对称式服务器集群资源利用率低的状况,我们提出了一种新的服务器集群实现方案一基于并行容错网的对等服务器集群。在服务器集群的实现中,我们采用了软件集群的方法,设计实现了服务器集群管理层和并行容错通信层。     

Web集群服务器可用性的提高

以Web集群服务器的后端节点作为研究对象，通过减少后端节点的MTTR（Mean Timeto Repair），来提高它们的可用性，从而提高整个集群服务器的可用性。首先，通过分析现有的故障恢复方案的不足，提出了新的改进方案，在新的方案中，采用了动态地检测和发送状态信息的策略。并引入了故障猜测状态，弥补了现有方案的不足。最后设计了一个试验环境，与现有的方案测试相比，使用改进的方案，MTTR（平均敞障修复时间）减少了63％，很好地提高了后端节点的可用性。     

为服务器保健——LANDesk服务器管理器8.6可实时评估服务器工作状态

让服务器保持健康状态一直是服务器管理的重中之重。借助服务器管理解决方案,企业可以优化服务器进程,大幅度缩短宕机时问。蓝代斯克软件公司最新版本的LANDesk服务器管理器8.6采用了软件许可证监控、操作系统部署以及蓝代斯克管理网关等新技术,使企业能借助日志合并、群组、软件许可证监控及更多功能,从单一控制台来管理不同厂商的硬件和操作系统,积极主动地进行容量规划和升级,最大程度地控制服务器,并智能地控制补丁、镜像和更新所需的自动化级别,实时评估服务器的工作状态,确保服务器高可用和高性能运行。主动监控提高可用性利用LAN…     

一个具有高可用性的动态容错VoD系统

提出了一个在VoD系统中具有高可用性的动态容错算法。针对分布式应用的特点，能对控制节点实现服务器切换的容错，数据服务节点负载均衡分配和故障节点提供服务的恢复。同时按照文章一致性保证机制可以实现媒体服务故障恢复对客户端的透明。通过对系统可用性分析和仿真试验表明，该算法可以有救地提高系统的可用性。     

基于马尔可夫过程模型的服务器集群数据可用性分析

可用性是衡量服务器集群的重要指标,在基于马尔可夫过程的服务器状态转移模型基础上，推出根据单台服务器的可用性计算服务器集群可用性的形式化方法，建立了单服务器可用性、服务单元可用性和整个服务器集群可用性之间的量化关系，进而得到多副本分布集群的数据可用性公式,本文的成果有效支持给定服务结点可用性参数的约束下准确估计服务器集群的可用性，对于从可用性角度辅助用户进行集群设计具有理论价值和实际意义．     

具有容侵能力的多播密钥管理方案

目前安全多播应用系统大多采用相对简单的集中式组密钥管理方案。单一密钥服务器容易导致单点失效和拥塞问题,系统安全性和可用性较差,恢复时延长。该文将入侵容忍的思想引入组密钥管理方案中,借助于门限秘密共享方案和信息分割算法,在集中式组密钥管理方法的基础上,提出一种具有容侵能力的组密钥管理方式。将单一的密钥服务器置换为分布式的密钥服务器组,各服务器借助于选举机制,通过相互协作来完成多播组密钥的管理以及信息的安全存储,既保持了集中式控制的优点,又有效减少了单点失效,增强了抗网络入侵的能力,提高了系统的可用性。     

基于团队机制的高可用性分布式系统的研究与实现

基于分布式环境的高可用系统是采用服务器冗余技术提高的可用性。本文针对分布式环境Ｃｏｒｂａ,给出了基于团队机制的高可用分布式系统的方案,结构和实现,并对这一方案的性能进行了分析和评价。     

嵌入式UDP服务器中数据报的构造

在嵌入式UDP服务器中，复制数据和计算检验和占据了处理分组的主要时间，提出了一种构造IP／UDP数据报的方案，并给出了数据报的VHDL构造算法．在服务器中采用FPGA并行处理数据，从而大大降低了UDP服务器分组的时间，该算法能很好地应用于嵌入式UDP服务器中，是一种很有实用价值的高速数据流处理算法．     

可用性增强的缩略图保持加密

传统加密在保护云储存图像隐私的同时使其完全丧失视觉可用性。尽管缩略图保持加密可平衡图像隐私性和视觉可用性，但现有方案均不考虑保留非视觉可用性。为此，提出一个可用性增强的缩略图保持加密方案。该方案利用信息隐藏的可逆性和灵活性，腾出原始图像的部分空间用于像素调整，从而保持密文缩略图与原始缩略图近似。在进行像素调整之前，对图像进行传统加密以保障安全性。在完成像素调整之后，凭借所剩余的空间容量嵌入额外信息，这些额外信息可为用户提供非视觉可用性。所提缩略图保持加密方案不仅平衡了隐私性和视觉可用性，还在一定程度上保留了非视觉可用性。最后，实验结果证实了所提方案的有效性和先进性。     

On Markov reward modelling with FSPNs

In this paper, fluid stochastic Petri nets (FSPNs) will be used for modelling reward in a performability model. Two variations of a known performability model are presented in order to demonstrate the ability of FSPNs in modelling accumulated rate reward as well as accumulated impulse reward. In the first model two fluid places are used, one of which represents the profit (reward) obtained by operating the system and the other one the buffer that is approximated continuously. In the second model only one fluid place is used, representing the costs (negative reward) arising due to repair of system components. The costs increase continuously at deterministic rate while the system is in state of repair (which is a rate reward in the model). Additional costs incur each time the buffer fails (which is an impulse reward in the model). With a numerical solution algorithm the distribution of the reward and its mean are computed. The accuracy of the numerical algorithm is studied by showing for the first model the impact of the choice of the discretization stepsizes on the obtained solution. Different boundary conditions are discussed for the second model.     

Performability analysis of guarded-operation duration: a translation approach for reward model solutions

Performability measures are often defined for analyzing the worth of fault-tolerant systems whose performance is gracefully degradable. Accordingly, performability evaluation is inherently well suited for application of reward model solution techniques. On the other hand, the complexity of performability evaluation for solving engineering problems may prevent us from utilizing those techniques directly, suggesting the need for approaches that would enable us to exploit reward model solution techniques through problem transformation. In this paper, we present a performability modeling effort that analyzes the guarded-operation duration for onboard software upgrading. More specifically, we define a “performability index” Y that quantifies the extent to which the guarded operation with a duration φ reduces the expected total performance degradation. In order to solve for Y, we progressively translate its formulation until it becomes an aggregate of constituent measures conducive to efficient reward model solutions. Based on the reward-mapping-enabled intermediate model, we specify reward structures in the composite base model which is built on three stochastic activity network reward models. We describe the model-translation approach and show its feasibility for design-oriented performability modeling.     

Performability assessment by model checking of Markov reward models

This paper describes efficient procedures for model checking Markov reward models, that allow us to evaluate, among others, the performability of computer-communication systems. We present the logic CSRL (Continuous Stochastic Reward Logic) to specify performability measures. It provides flexibility in measure specification and paves the way for the numerical evaluation of a wide variety of performability measures. The formal measure specification in CSRL also often helps in reducing the size of the Markov reward models that need to be numerically analysed. The paper presents background on Markov-reward models, as well as on the logic CSRL (syntax and semantics), before presenting an important duality result between reward and time. We discuss CSRL model-checking algorithms, and present five numerical algorithms and their computational complexity for verifying time- and reward-bounded until-properties, one of the key operators in CSRL. The versatility of our approach is illustrated through a performability case study.     

一种基于含水印量子图像的自适应量子隐写算法

利用NEQR量子图像表示法,提出了一种能在含水印量子载体图像中实现隐蔽通信的量子隐写算法。新算法借助水印通常拥有很好的稳健性和其特有的自恢复系统,对秘密信息的稳健性进行了多重强化。相比于之前的量子隐写算法,新算法不仅强化了秘密信息自身的稳健性,而且通过量子线路的设计提高了其嵌入和提取的可执行性和执行效率。经实验仿真结果和性能分析验证,新算法在保留原有隐蔽性和安全性基础上,进一步提高了秘密信息的稳健性和嵌入率。     

Performability evaluation of emergency call center

Emergency call centers serve people in utmost circumstances; hence they should be highly dependable. Availability and performance are key aspects in call centers. Architectural models, service policies, and redundancies are attributes for defining and evaluating the overall operation of such systems. In this work, we performed the performability evaluation of an important emergency call center located in a large city in Brazil. The results showed that the system’s downtime is considered high, so, we proposed a new architecture to improve the system’s performability. The results obtained in this paper can be used to provide support for decisions on interventions in the emergency call center to improve its performability. It is expected that the models presented in this paper will be useful in a variety of emergency call centers.     

A new computational algorithm for retrial queues to cellular mobile systems with guard channels

Retrial queueing models have been applied to evaluate the impact of new and handover calls on the call control mechanisms of cellular mobile networks. The fact that the retrial rate depends on the number of retried calls waiting in the system leads to an analytically intractable model. Therefore, approximation procedures should be used to compute the performability of the system. However, there exists a numerical problem concerning a recursive algorithm to compute the stationary probabilities for retrial queues modeling guard channels. Namely, the consideration of guard channels leads to calculations with negative terms in the recursive algorithm. Negative terms and extremely small values involved in the computation are the main causes for a numerical instability in the recursive algorithm.     

Quantifying the performability of cluster-based services

In this paper, we propose a two-phase methodology for systematically evaluating the performability (performance and availability) of cluster-based Internet services. In the first phase, evaluators use a fault-injection infrastructure to characterize the service's behavior in the presence of faults. In the second phase, evaluators use an analytical model to combine an expected fault load with measurements from the first phase to assess the service's performability. Using this model, evaluators can study the service's sensitivity to different design decisions, fault rates, and other environmental factors. To demonstrate our methodology, we study the performability of a multitier Internet service. In particular, we evaluate the performance and availability of three soft state maintenance strategies for an online bookstore service in the presence of seven classes of faults. Among other interesting results, we clearly isolate the effect of different faults, showing that the tier of Web servers is responsible for an often dominant fraction of the service unavailability. Our results also demonstrate that storing the soft state in a database achieves better performability than storing it in main memory (even when the state is efficiently replicated) when we weight performance and availability equally. Based on our results, we conclude that service designers may want an unbalanced system in which they heavily load highly available components and leave more spare capacity for components that are likely to fail more often.     

An efficient solution to a retrial queue for the performability evaluation of DHCP

The Dynamic Host Configuration Protocol (DHCP) is designed to provide an automatic mechanism for the allocation, configuration and management of IP addresses and TCP/IP protocol stack parameters of computers and devices in IP networks. The important feature of DHCP is a “dynamic allocation” mechanism, which assigns an IP address to a client for a limited period of time (called a lease time). Therefore, a previously allocated IP address can automatically be assigned to another host by a DHCP server upon the expiration of the lease time if a host does not renew the use of an allocated IP address.This paper proposes a retrial queueing model to approximate the performability of the DHCP dynamic allocation mechanism. An efficient computational algorithm is developed to calculate the steady state probabilities of a continuous time discrete state Markov process. A comparison of our approximate model with a detailed simulation model of the DHCP dynamic allocation mechanism shows an excellent agreement between the analytical and the simulation results. The impact of the lease time parameter on the performability of the DHCP dynamic allocation mechanism is also illustrated through a numerical study.     

Behavioral-based performability modeling and evaluation of e-commerce systems

Assuring high quality of web services, especially regarding service reliability, performance and availability of e-commerce systems (unified under the term performability), has turned into an imperative of the contemporary way of doing business on the Internet. Recognizing the fact that customers’ online shopping behavior is largely affecting the conduct of e-commerce systems, the paper promotes a customer-centric, holistic approach: customers are identified as the most essential “subsystem” with a number of important, but less well-understood behavioral factors. The proposed taxonomy of customers and the specification of operational profiles is a basis to building predictive models, usable for evaluating a range of performability measures. The hierarchical composition of sub-models utilizes the semantic power of deterministic and stochastic Petri nets, in conjunction with discrete-event simulation. A handful of variables are identified in order to turn performability measures into business-oriented performance metrics, as a cornerstone for conducting relevant server sizing activities.     

基于保能性的层次化系统可生存性评估方法

随着网络和分布式系统在关键基础设施中的广泛应用,人们对于保能性的关注程度越来越高。基于保能性,参考了传统的可生存系统分析方法和概率风险评测方法,采用层次化方法,将可生存性归结到带来可生存性影响的不利事件上,然后从技术角度和经济效益角度考虑可生存性量化。