低编译复杂度的双容错阵列码

纠删码技术是独立磁盘冗余阵列-6(RAID-6)的双容错能力的底层实现技术，它的性能是左右RAID-6性能的重要因素。针对RAID-6中常用阵列纠删码的I/O不平衡和数据恢复速度慢的问题，提出一种基于异或（XOR）的混合阵列码——J码（J-code）。J-code采用新的校验生成规则，首先，利用原始数据构造的二维阵列计算出对角校验位并构造新的阵列；然后，利用新阵列中数据块之间的位置关系计算得到反对角校验位。此外，J-code将原始数据与部分校验位存储于同一磁盘，能减少编译码过程中的异或（XOR）操作次数和单盘恢复过程中读取数据块的个数，从而降低编译码复杂度和单盘故障修复的I/O成本，缓解磁盘热点集中现象。仿真实验结果表明，相较于RDP(RowDiagonal Parity)、EaR(Endurance-aware RAID-6)等阵列码，J-code的编码时间减少了0.30%～28.70%，单磁盘故障和双磁盘故障的修复用时分别减少了2.23%～31.62%和0.39%～36.00%。     

基于流命令的SCSI目标端设计

介绍一种基于SCSI流命令的SCSI的目标端,可以将接收到的流命令转换成针对SCSI磁盘的块传输命令。为了得到更好的传输性能,在该目标端中实现了缓存机制。分析了缓存对传输速度的影响,建立了SCSI目标端的传输模型,给出了仿真结果和实验结果。     

iSCSI目标器的实现

SCSI是一种支持块级别访问的通用磁盘技术,但主要缺点是SCSI总线的距离限制.iSCSI协议使用以太网传输SCSI命令,因而可以有效地解决SCSI的扩展性.研究iSCSI的协议,并提出iSCSI目标器的实现模型.     

基于简单再生码的分段编码方案

简单再生码将可容多错的RS纠删码与简单的异或运算相结合,在达到容忍任意n－k个节点故障可靠性的基础上,可以实现对单个失效节点的高效快速修复。对简单再生码的失效节点修复过程进行改进,提出一种新的基于简单再生码的分段编码方案,将f个具有相同下标的编码块分成两段,将每段中的编码块进行异或操作,生成一个新的校验块。对该方案的存储开销、磁盘读取的开销以及修复带宽开销进行性能分析和仿真实验,结果表明提出的基于简单再生码的分段编码方案在增加少量存储开销的同时,其修复带宽和磁盘读取的开销性能有了很大程度的优化,进一步验证了改进方案的正确性和有效性。     

一种优化的小写操作RAID6算法

提出一种优化的小写操作RAID6算法XP-Code。该算法按对角线和逆对角线划分校验组,每个校验组中有一个校验块和N-2个数据块,校验块均匀分布在2条主对角线上。由于使用MDS编码,XP-Code只需XOR计算,且使用公式推导,实现简单。理论分析及与其他典型RAID6算法的比较表明,XP-Code的校验块计算、丢失数据恢复和小写操作等操作的效率都是最优的。     

“异或”校验漏检率分析

“异或”校验(Block Check Character)在使用中存在错误无法检测即漏检的发生，其可靠性未得到理论层面的证明。为证明“异或”校验的可靠性，通过对“异或”校验的校验过程进行分析，得到了“异或”校验发生漏检的原理并找到了漏检出现的根本原因。采用数学分析的方法得到了影响漏检率的因素是校验位长度与所校验数据量，推导出“异或”校验的漏检率计算公式。最后将计算公式应用在北斗短报文通信这一重要实际应用中，得到了北斗短报文“异或”校验位的漏检率在0.003 90～0.003 91范围内，证明了“异或”校验在短报文通信应用中的可靠性。     

一种基于3容错阵列码的RAID数据布局

在EVENODD码的基础上,提出一种新的基于EEOD码的RAID数据布局,只需要3个额外的磁盘保存校验信息,能容许任意3个磁盘同时故障,并给出了EEOD的代数定义,理论上证明了EEOD码的MDS性质.从一种新的途径讨论了EEOD码的译码过程:用图的回路表示通过"异或"运算得到的校验方程组,把译码过程归结为图回路的叠加,进而校验方程组图中度为偶数的顶点逐步消除.讨论了基于EEOD码阵列布局的性能,与其它RAID结构相比,容灾能力大幅度提高,编码和译码过程只需要简单的异或运算,但是空间利用率影响非常小,并且EEOD具有很好的性能,具有很好的应用前景.     

利用DCMT来解决RAID5的小写问题

在盘阵中，RAID5利用校验信息来提高数据的可靠性。由于要维护校验信息，带来了小写问题，影响了系统的整体性能。本文在AFRAID方法的基础上，提出了一种基于动态缓存标志表（DCMT）的提高RAID5性能的方法。该方法在保证RAID5磁盘数据特征的前提下，用较小的代价就可以大大缩短响应时间，提高系统整体性能。     

一种基于RAID5的Disk Cache的实现

介绍了一种基于RAID5的Disk Cache的实现。在对磁盘阵列Cache的实现过程中，使用了组相联映射、LRU替换算法等比较成熟的技术，在Cache回写策略上采用write-back方式。从而提高了写磁盘速度，减少冗余写盘操作。另外通过对校验组加锁，有效地防止了同一校验组里多个块同时降级而导致的数据不一致现象。     

PRCD技术及其软件实现

介绍了一种新的磁盘存储体系结构PRCD（prefetching RAM caching disk,预取主存缓冲磁盘技术）。该技术通过收集小写转换成为一次大写请求提高系统的写性能，并采用数据块预取技术大大提高系统的读性能。还介绍了基于Windows NT平台的PRCD的软件实现方法。     

Reconstruct versus read-modify writes in RAID

RAID5 (Redundant Arrays of Independent Disk level 5) is a popular paradigm, which uses parity to protect against single disk failures. A major shortcoming of RAID5 is the small write penalty, i.e., the cost of updating parity when a data block is modified. Read-modify writes and reconstruct writes are alternative methods for updating small data and parity blocks. We use a queuing formulation to determine conditions under which one method outperforms the other. Our analysis shows that in the case of RAID6 and more generally disk arrays with k check disks tolerating k disk failures, RCW outperforms RMW for higher values of N and G. We note that clustered RAID and variable scope of parity protection methods favor reconstruct writes. A dynamic scheme to determine the more desirable policy based on the availability of appropriate cached blocks is proposed.     

一种新型瓦记录磁盘的高可靠数据存储方法

近年来,传统磁记录的存储密度增长已经达到极限,为了满足快速增长的数据容量需求,多种新型存储技术不断涌现,其中瓦记录（shingled magnetic recording,SMR）技术已实现商业化,在企业实际应用.由于瓦记录磁盘的叠瓦式结构,磁盘在随机写入时会引起写放大,造成磁盘性能下降.这一问题在部署传统的高可靠存储方案（如RAID5）时会变得更加严重,原因在于校验数据更新频率很高,磁盘内出现大量的随机写请求.研究发现瓦记录内部其实存在具有原位更新能力的“可覆盖写磁道（free track）”,基于“可覆盖写磁道”,提出了一种专门针对瓦记录盘的高可靠数据存储方法——FT-RAID,以替代经典的RAID5方法,实现一种廉价、大容量、高可靠的存储系统.FT-RAID包含两个部分：“可覆盖写磁道映射（FT-mapping）”和“可覆盖写磁道缓冲区（FT-buffer）”.FT-mapping实现了一种瓦记录友好的RAID映射方式,将频繁更新的校验块数据映射至“可覆盖写磁道”;FT-buffer实现了一种瓦记录友好的两层缓冲区结构,上层确保了热数据能够原位更新,下层提高了缓冲区的容量.基于真实企业I/O访问记录的实验结果表明,与传统RAID 5相比,FT-RAID能够减少80.4%的写放大率,显著提高存储系统整体性能.     

基于IOP321的嵌入式RAID系统中的硬异或实现

为提高RAID系统的可靠性,RAID3/5/6算法在数据写入的过程中采用异或运算产生奇偶校验信息。当RAID出现故障时,也是通过异或运算完成数据的重构。因此,异或运算是RAID系统工作时频繁而且重要的操作之一。本文详细介绍了采用Intel IOP321处理器的应用加速单元实现异或运算的工作原理和软件模块设计,并通过实验测试证明,专门的硬异或单元比嵌入式处理器做软件异或运算的速度快7倍以上,有效地解决了嵌入式环境下异或的性能瓶颈问题。     

Ripple-RAID:一种面向连续数据存储的高效能盘阵

视频监控、备份、归档等应用具有独特的负载特性和I/O访问模式,需研究特定的存储节能方法.磁盘阵列的局部并行策略有利于实现该类存储系统的节能,但通常会导致RAID执行小写操作而严重影响性能.为此,提出一种面向该类存储系统的高效能盘阵——Ripple-RAID,采用新的局部并行数据布局,通过综合运用地址转换、异地更新、基于流水技术渐进生成校验、分段数据恢复等策略,在单盘容错条件下,保持了局部并行的节能性,又有效解决了局部并行带来的小写问题.Ripple-RAID具有突出的性能和节能效率,在80%顺序写负载情况下,请求长度为512KB时,写性能为S-RAID 5的3.9倍,Hibernator、MAID写性能的1.9倍,PARAID、eRAID 5写性能的0.49倍;而比S-RAID 5节能20%,比Hibernator、MAID节能33%,比eRAID 5节能70%,比PARAID节能72%.     

基于块I/O的RAID设计

磁盘阵列(RAID)是当前能够提供存储系统高可用性和高可靠性的一项重要技术。它通过软硬件的冗余和奇偶校验提供数据的重构和恢复。针对当前在RAID控制软件设计的过程中面临多次数据拷贝的问题,文中提出了一种基于块I/O的RAID系统设计。它利用最新的Linux内核所提供的BIO机制,插入到SCSI Target的中间层进行数据I/O的处理。它能屏蔽掉上层不同的设备驱动类型,提供到IP-SAN的无缝链接。实验表明,该设计能够减少数据的传输延迟,最大限度地提高数据传输过程中的吞吐率,避免了多次昂贵的内存拷贝操作。     

A high-performance and endurable SSD cache for parity-based RAID

Solid-state drives (SSDs) have been widely used as caching tier for disk-based RAID systems to speed up dataintensive applications. However, traditional cache schemes fail to effectively boost the parity-based RAID storage systems (e.g., RAID-5/6), which have poor random write performance due to the small-write problem. What’s worse, intensive cache writes can wear out the SSD quickly, which causes performance degradation and cost increment. In this article, we present the design and implementation of KDD, an efficient SSD-based caching system which Keeps Data and Deltas in SSD. When write requests hit in the cache, KDD dispatches the data to the RAID storage without updating the parity blocks to mitigate the small write penalty, and compactly stores the compressed deltas in SSD to reduce the cache write traffic while guaranteeing reliability in case of disk failures. In addition, KDD organizes the metadata partition on SSD as a circular log to make the cache persistent with low overhead.We evaluate the performance of KDD via both simulations and prototype implementations. Experimental results show that KDD effectively reduces the small write penalty while extending the lifetime of the SSD-based cache by up to 6.85 times.     

A reliable and energy-efficient storage system with erasure coding cache

In modern energy-saving replication storage systems, a primary group of disks is always powered up to serve incoming requests while other disks are often spun down to save energy during slack periods. However, since new writes cannot be immediately synchronized into all disks, system reliability is degraded. In this paper, we develop a high-reliability and energy-efficient replication storage system, named RERAID, based on RAID10. RERAID employs part of the free space in the primary disk group and uses erasure coding to construct a code cache at the front end to absorb new writes. Since code cache supports failure recovery of two or more disks by using erasure coding, RERAID guarantees a reliability comparable with that of the RAID10 storage system. In addition, we develop an algorithm, called erasure coding write (ECW), to buffer many small random writes into a few large writes, which are then written to the code cache in a parallel fashion sequentially to improve the write performance. Experimental results show that RERAID significantly improves write performance and saves more energy than existing solutions.     

Exploiting flash memory characteristics to improve performance of RAIS storage systems

Redundant array of independent SSDs (RAIS) is generally based on the traditional RAID design and implementation. The random small write problem is a serious challenge of RAIS. Random small writes in parity-based RAIS systems generate significantly more pre-reads and writes which can degrade RAIS performance and shorten SSD lifetime. In order to overcome the well-known write-penalty problem in the parity-based RAID5 storage systems, several logging techniques such as Parity Logging and Data Logging have been put forward. However, these techniques are originally based on mechanical characteristics of the HDDs, which ignore the properties of the flash memory. In this article, we firstly propose RAISL, a flash-aware logging method that improves the small write performance of RAIS storage systems. RAISL writes new data instead of new data and pre-read data to the log SSD by making full use of the invalid pages on the SSD of RAIS. RAISL does not need to perform the pre-read operations so that the original characteristics of workloads are kept. Secondly, we propose AGCRL on the basis of RAISL to further boost performance. AGCRL combines RAISL with access characteristic to guide read and write cost regulation to improve the performance of RAIS storage systems. Our experiments demonstrate that the RAISL significantly improves write performance and AGCRL improves both of write performance and read performance. AGCRL on average outperforms RAIS5 and RAISL by 39.15% and 16.59% respectively.