闪存环境下B+树索引重访

闪存容量的增大使在其上构建大型系统成为可能,如何构建闪存数据库也成为数据库的热点研究领域之一。索引结构是数据库中必不可少的结构之一,而B+树是最广泛使用的索引结构。这里对存储在闪存芯片模拟器及固态硬盘上的B+树性能进行了测试及分析。首先介绍了闪存的IO特点,并测试了固态硬盘的基本IO特性。接着,对B+树的插入和查询效率进行了详细地测试。测试发现节点大小,缓存大小,以及数据值的分布方式都会对B+树的性能带来很大影响。例如由于闪存的读取速度不对称,闪存的更新和查询操作最优块大小相差较大。这些测试结果为更好地在闪存上使用B+树索引,并进一步设计出更适合闪存的索引提供了指导。     

基于哈希的B+树结构的频繁模式挖掘

提出了一种基干改进的B 树结构及一种新的数据挖掘算法,HB-Minc,该算法通过构造哈希函数,获得B 树的关键字,并在B 树的叶子结点上构建链表结构,记录卡H关关键字的项集及频数,这样在无需产生巨大的候选项集的情况下,挖掘出频繁模式,且具有较高的时间效率。     

一种新的基于B+树结构索引的XML元素的连接算法

该文通过对传统的NumberingSchema进行改进,并结合B+树提出了一种新的索引———B+树结构索引。在B+树结构索引的基础上提出了一种有效的连接算法,该算法通过削减不参加连接的元素来实现快速、有效的连接。     

基于CB+-tree的时态XML索引

针对时态查询与时间属性紧密相关的特点,利用时间区间作为改进后B+-tree的索引关键字建立索引,改进后的B+-tree命名为Changing B+-tree(CB+-tree)。实验证明,在CB+-tree上进行时态查询比B+-tree及基于DOM的XML文档的查询效率有所提高。     

隐文加密系统中的隐文B+树加密索引方法

传统方法使用对文件或整个磁盘进行加密来保护数据信息，加密后的信息本身无法隐藏自身的存在，文件本身的存在会促使感兴趣的攻击者破解加密信息。在某些情况下攻击者可以使用强迫手段迫使文件的主人公开他的加密密钥，这使得复杂的加密技术变得毫无意义。该文介绍了一种着重于隐藏数据存在性的加密手段——隐文加密，对重要文件的存储进行加密，该方法采用隐文B+树建立Unix系统文件索引，通过隐藏表将隐文B+树索引隐蔽起来，使攻击者无法知道重要信息的存在，保障了信息的安全性。     

一种新的基于B~+树结构的XML元素的索引方法

提出了一种用于搜索XML文档的新的索引方法即RIST。通过采用代码化的结构序列(SES)来表示XML文档和XML查询,得出查询XML数据等同于查找子序列匹配。RIST采用树结构作为查询的基本单元,从而避免了代价高昂的连接操作。另外,RIST还在XML文档的内容和结构上提供了一个统一的索引,所以它的一个很明显的优势就是克服了仅仅根据内容或结构建立索引的弊端。实验表明RIST在支持结构查询上是一种高效的方法。     

Flash-Optimized B+-Tree

With the rapid increasing capacity of flash memory, flash-aware indexing techniques are highly desirable for flash devices. The unique features of flash memory, such as the erase-before-write constraint and the asymmetric read/write cost, severely deteriorate the performance of the traditional B+-tree algorithm. In this paper, we propose an optimized indexing method, called lazy-update B+-tree, to overcome the limitations of flash memory. The basic idea is to defer the committing of update requests to the B...     

面向新型非易失存储的文件系统研究综述

新型非易失存储(NVM)可字节寻址,具有近似内存的低延迟特性以及外存的非易失性,受限于软硬件技术成熟度,目前首先被用于外存.讨论了NVM用于持久性外存所面临的一系列问题,以及管理上的一些挑战;对现有的典型NVM文件系统及其主要特性进行了梳理.归纳起来,这些特性主要围绕降低一致性开销、降低软件栈开销、内存与外存的融合、分布式文件系统、NVM文件系统安全、容错、空间管理几个方面展开.最后,展望了NVM文件系统仍然有待探讨的几个研究方向,包括扩展性问题、虚拟内存与文件系统的有机融合以及分布式文件系统等.     

FQM-tree:基于B~+树的时态索引

提出了一种基于B~+树的、针对有效时间区间的索引模式:FQM-tree(fast query map tree).FQM-tree将有效时间区间映射为一维空间上的点,对映射点建立索引;同时增加一个基于内存的辅助索引结构,最大程度地减少对无效节点的访问.该索引结构具有如下优点:首先,将时问区间映射为点,可以直接利用已有应用成熟的、被广泛支持索引结构(如B~+树),这就意味着可以在数据库中快速实现对时态索引的支持,而无需更改DBMS的内核;其次,基于内存的辅助索引结构可以直接定位叶节点,提高了时态查询效率;另外,由于当前B~+树仍是数据库中应用最成熟、最广泛的索引结构,因此借助B+树的基本结构研究时态索引,使得时态索引能够快速地得到具体应用,所以FQM-tree的研究具有广泛的应用价值.理论分析及大量的实验结果均表明,FQM-tree的查询性能优于传统的时态索引方法.     

Write-Optimized B+ Tree Index Technology for Persistent Memory

Due to its low latency,byte-addressable,non-volatile,and high density,persistent memory (PM) is expected to be used to design a high-performance storage system.However,PM also has disadvantages such as limited endurance,thereby proposing challenges to traditional index technologies such as B+ tree.B+ tree is originally designed for dynamic random access memory (DRAM)-based or disk-based systems and has a large write amplification problem.The high write amplification is detrimental to a PM-based system.This paper proposes WO-tree,a write-optimized B+ tree for PM.WO-tree adopts an unordered write mechanism for the leaf nodes,and the unordered write mechanism can reduce a large number of write operations caused by maintaining the entry order in the leaf nodes.When the leaf node is split,WO-tree performs the cache line flushing operation after all write operations are completed,which can reduce frequent data flushing operations.WO-tree adopts a partial logging mechanism and it only writes the log for the leaf node.The inner node recognizes the data inconsistency by the read operation and the data can be recovered using the leaf node information,thereby significantly reducing the logging overhead.Furthermore,WO-tree adopts a lock-free search for inner nodes,which reduces the locking overhead for concurrency operation.We evaluate WO-tree using the Yahoo!Cloud Serving Benchmark(YCSB) workloads.Compared with traditional B+ tree,wB-tree,and Fast-Fair,the number of cache line flushes caused by WO-tree insertion operations is reduced by 84.7％,22.2％,and 30.8％,respectively,and the execution time is reduced by 84.3％,27.3％,and 44.7％,respectively.     

面向非易失内存的异构索引

非易失内存(non-volatile memory,NVM)为数据存储与管理带来新的机遇,但同时也要求已有的索引结构针对NVM的特性进行重新设计.围绕NVM的存取特性,重点研究了树形索引在NVM上的访问、持久化、范围查询等操作的性能优化,并提出了一种上下两层结构的异构索引HART.该索引结合了B+树与Radix树的特点...     

Indexing XML documents for XPath query processing in external memory

Existing encoding schemes and index structures proposed for XML query processing primarily target the containment relationship, specifically the parent–child and ancestor–descendant relationship. The presence of preceding-sibling and following-sibling location steps in the XPath specification, which is the de facto query language for XML, makes the horizontal navigation, besides the vertical navigation, among nodes of XML documents a necessity for efficient evaluation of XML queries. Our work enhances the existing range-based and prefix-based encoding schemes such that all structural relationships between XML nodes can be determined from their codes alone. Furthermore, an external-memory index structure based on the traditional B+-tree, XL+-tree(XML Location+-tree), is introduced to index element sets such that all defined location steps in the XPath language, vertical and horizontal, top-down and bottom-up, can be processed efficiently. The XL+-trees under the range or prefix encoding scheme actually share the same structure; but various search operations upon them may be slightly different as a result of the richer information provided by the prefix encoding scheme. Finally, experiments are conducted to validate the efficiency of the XL+-tree approach. We compare the query performance of XL+-tree with that of R-tree, which is capable of handling comprehensive XPath location steps and has been empirically shown to outperform other indexing approaches.     

UCat: heterogeneous memory management for unikernels

Unikernels provide an efficient and lightweight way to deploy cloud computing services in application-specialized and single-address-space virtual machines (VMs). They can efficiently deploy hundreds of unikernel-based VMs in a single physical server. In such a cloud computing platform, main memory is the primary bottleneck resource for high-density application deployment. Recently, non-volatile memory (NVM) technologies has become increasingly popular in cloud data centers because they can offer extremely large memory capacity at a low expense. However, there still remain many challenges to utilize NVMs for unikernel-based VMs, such as the difficulty of heterogeneous memory allocation and high performance overhead of address translations. In this paper, we present UCat, a heterogeneous memory management mechanism that support multi-grained memory allocation for unikernels. We propose front-end/back-end cooperative address space mapping to expose the host memory heterogeneity to unikernels. UCat exploits large pages to reduce the cost of two-layer address translation in virtualization environments, and leverages slab allocation to reduce memory waste due to internal memory fragmentation. We implement UCat based on a popular unikernel--OSv and conduct extensive experiments to evaluate its efficiency. Experimental results show that UCat can reduce the memory consumption of unikernels by 50% and TLB miss rate by 41%, and improve the throughput of real-world benchmarks such as memslap and YCSB by up to 18.5% and 14.8%, respectively.     

基于新型存储器件的分布式文件系统性能优化

新型存储器件的I/O性能通常比传统固态驱动器（SSD）高一个数量级,然而使用新型存储器件的分布式文件系统相对于使用SSD的分布式文件系统性能并没有显著的提高,这说明目前的分布式文件系统并不能充分发挥新型存储器件的性能。针对这个问题,对Hadoop分布式文件系统（HDFS）的数据写入流程及传输过程进行了量化分析。通过量化分析HDFS数据写入过程各阶段的时间开销,发现在写入数据的各个阶段中,节点间数据传输的时间占比较大。因此提出了对应的优化方案,通过异步写入的方式并行化数据传输与处理过程,使得不同数据包的处理阶段叠加起来,减少了数据包整体的处理时间,从而提升了HDFS的写入性能。实验结果表明,所提方案将HDFS的写入吞吐量提升了15%~24%,总体的写入执行时间降低了28%~36%。     

基于B+树的闪存文件系统实现方案

随着闪存容量的快速增大,JFFS2文件系统暴露出了挂载时间长、可移植性差等缺点,JFFS3提出将文件索引信息保存在闪存上的设计方法,其中将整个文件系统组织成一个大的B+树是提高文件系统性能和可移植性的关键点。该文给出一种基于JFFS3的设计思想并引入区域管理方式的实现方案。     

相变存储器写寿命延长关键技术研究进展

随着大数据分析应用时效性提升和“存储墙”问题日益突出,存储系统已成为当前计算机系统整体性能的瓶颈。以相变存储器（PCM）为代表的新型非易失性存储器（NVM）具有集成度高、功耗低、读写访问速度高、非易失、体积小和抗震等优良特性,已成为最具潜力的下一代存储设备。然而,写寿命有限是PCM实用化的一道障碍,如何通过减少写操作和磨损均衡以提升PCM使用寿命是当前的研究热点。 从减少PCM写操作、均匀写操作分布以及在混合内存中的页面迁移等三个方面介绍了当前PCM写寿命延长技术的研究现状以及优缺点,最后探讨未来进一步改进PCM寿命可能的研究方向。     

B+树索引机制的研究及优化

B+树索引是大型数据库系统中较为常用的一种索引技术。当数据庞杂时,B+树索引在查找效率和空间利用率方面还存在不足。针对该问题提出一种改进的B+树结构,首先通过调整叶子结点与非叶子结点的数量关系,以降低树的深度;然后优化原插入算法,在分裂结点前进行平衡处理(BP),以提高树的空间利用率。经试验,改进后的B+树与传统B+树相比,在查找效率和空间利用率上分别提高了10%和6%,证明对B+树的改进具有可行性。     

Upscaledb: Efficient integer-key compression in a key-value store using SIMD instructions

Compression can sometimes improve performance by making more of the data available to the processors faster. We consider the compression of integer keys in a B+-tree index. For this purpose, systems such as IBM DB2 use variable-byte compression over differentially coded keys. We revisit this problem with various compression alternatives such as Google's VarIntGB, Binary Packing and Frame-of-Reference. In all cases, we describe algorithms that can operate directly on compressed data. Many of our alternatives exploit the single-instruction-multiple-data (SIMD) instructions supported by modern CPUs. We evaluate our techniques in a database environment provided by Upscaledb, a production-quality key-value database. Our best techniques are SIMD accelerated: they simultaneously reduce memory usage while improving single-threaded speeds. In particular, a differentially coded SIMD binary-packing techniques (BP128) can offer a superior query speed (e.g., 40% better than an uncompressed database) while providing the best compression (e.g., by a factor of ten). For analytic workloads, our fast compression techniques offer compelling benefits. Our software is available as open source.     

重定向内存资源B+树索引文件缓冲区策略

在作者研究的鉴定古陶瓷的专家系统（Expert System for Appraising Pottery and Porcelain， ESAPP）中，需要使用一个大型的数据库管理系统，如何合理而有效地使用内存资源，是进行系统设计时需要考虑的一个重要因素。在该系统中，设计了一种“可重定向内存资源的B+树索引文件缓冲区管理策略”，它把文件缓冲区中的内存页面定义为一种可以重定义的系统资源，极大地提高了系统的运行效率。     

良性蠕虫的B+地址树扩散策略

为提高良性蠕虫对抗恶意蠕虫时的效率,提出一种良性蠕虫的B+地址树扩散策略。在传递的信息中带有控制信息,主动扩散并行信息,以便使良性蠕虫处于可控的扩散状态。根据B+地址树设计动态生成算法,利用Scilab进行扩散模拟测试。测试结果表明,与已有策略相比,该策略的良性蠕虫扩散速度更快,对网络的影响更小。