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arXiv 1807.02253cs.DCcs.ITmath.IT
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大规模系统中的快速数据访问:一般服务时间分布下的网络延迟分析

Faster Data-access in Large-scale Systems: Network-scale Latency Analysis under General Service-time Distributions

Avishek Ghosh, Kannan Ramchandran

摘要:

在具有大量服务器的云存储系统中,文件通常不存储在单个服务器中。相反,它们被拆分、复制(以确保在服务器故障时的可靠性)并存储在不同的服务器中。我们分析了这样一个分裂和复制的云存储系统的平均延迟下一般的次指数服务时间。我们提出了一种新的调度方案,利用负载平衡策略的\textit{power of $d$ $(\geq 2)$}的选择。分割和复制的替代方案是使用擦除码,最近,已经观察到它们可以减少数据访问中的延迟(有关详细信息,请参见\cite{longbo_delay})。我们认为,在高冗余(整数冗余因子严格大于或等于2)制度下,编码系统的平均延迟是上界的分裂和复制系统(具有相同的复制因子),这两者之间的差距是小的。我们验证了这一说法在不同的服务分布,如指数,移位加指数和重尾Weibull分布下的数值和比较的平均延迟的unsplit-replicated系统。我们观察到,编码系统优于未分割复制系统至少$20\%$。此外,我们认为,平均延迟的纠删编码系统具有低冗余(分数冗余因子1和2之间),这是一个更务实的情况下,给定的存储约束(\cite{rashmi_thesis})。然而,在这种制度下,我们限制自己的特殊情况下,指数的服务时间分布,并使用随机负载均衡策略,即\textit{batch-sampling}。我们得到的上界平均延迟,取决于顺序统计的队列长度,其中,我们进一步平滑通过离散到连续的近似。

英文摘要:

In cloud storage systems with a large number of servers, files are typically not stored in single servers. Instead, they are split, replicated (to ensure reliability in case of server malfunction) and stored in different servers. We analyze the mean latency of such a split-and-replicate cloud storage system under general sub-exponential service time. We present a novel scheduling scheme that utilizes the load-balancing policy of the \textit{power of $d$ $(\geq 2)$} choices. An alternative to split-and-replicate is to use erasure-codes, and recently, it has been observed that they can reduce latency in data access (see \cite{longbo_delay} for details). We argue that under high redundancy (integer redundancy factor strictly greater than or equal to 2) regime, the mean latency of a coded system is upper bounded by that of a split-and-replicate system (with same replication factor) and the gap between these two is small. We validate this claim numerically under different service distributions such as exponential, shift plus exponential and the heavy-tailed Weibull distribution and compare the mean latency to that of an unsplit-replicated system. We observe that the coded system outperforms the unsplit-replication system by at least $20\%$. Furthermore, we consider the mean latency for an erasure coded system with low redundancy (fractional redundancy factor between 1 and 2), a scenario which is more pragmatic, given the storage constraints (\cite{rashmi_thesis}). However under this regime, we restrict ourselves to the special case of exponential service time distribution and use the randomized load balancing policy namely \textit{batch-sampling}. We obtain an upper bound on mean delay that depends on the order statistics of the queue lengths, which, we further smooth out via a discrete to continuous approximation.

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