一种高性能高可靠的混合客户端缓存系统

现代数据中心普遍使用网络存储系统提供共享存储服务.存储服务端通常使用独立冗余磁盘阵列(RAID)技术保障数据可靠性,如可以容单/双盘错的RAID5/6.相比于传统磁盘,固态盘具有更低的访问时延和更高的价格,因此将固态盘作为存储客户端缓存成为一种流行的方案.写回法可以充分发挥固态盘的优势加速存储读写性能,然而一旦固态盘发生故障,写回法无法保证数据的一致性和持久性.写直达法简化了一致性模型,但是无法减小写时延.设计并实现一种新的混合客户端缓存(hybrid host cache, HHC),HHC通过使用廉价的日志磁盘镜像存放固态盘上的脏数据来提高可靠性,并且利用写屏障语义保证数据的可靠性和一致性.分析表明,HHC的平均无故障时间远远高于后端存储系统.最后实现了一个原型系统并使用Filebench进行性能评估,结果表明在不同负载下,HHC性能与传统的写回法接近,远远超过写直达法.     

组合盘中结合访问次数与能量代价的文件缓存替换策略

如何有效地降低存储子系统能耗是近几年研究的热点议题。新型非易失、抗震、低功耗闪存及固态盘的出现给存储子系统节能带来了新的机会。但其每单位价格昂贵,目前难以替代硬盘的角色。结合硬盘和固态盘的优势,本文采用组合盘(由硬盘和固态盘组成)节能。结合文件访问次数和能量代价,我们提出了改进的文件缓存替换策略FEBR(Frequency&Energy-based replacement)。实验采用两个真实办公用户数据,结果表明组合盘方案是可行的,节能百分比可达70%～80%;与经典替换算法、最新较好的ARC算法以及理想最优页面OPT算法进行了详细比较,FEBR优于其它策略。     

优化山寨固态硬盘，让其也有好性能

山寨固态硬盘以其价格低廉的特性从一定程度匕改善了固态硬盘最大的缺点——价格。但是无缓存、读写不稳定、随机写入惨不忍睹等缺点也让人头痛不已。     

闪存、主控、缓存 固态硬盘最重要的三颗芯

SSD固态硬盘的普及,终于解决了Pc在存储性能上的短板。那么,这种依托于闪存技术的存储设备源何这般强劲？其背后又有哪些值得我们关注的技术？本文我们就以闪存领域领导品牌——闪迪旗下产品为例,探讨一下影响SSD性能表现的关键所在。 固态硬盘的三颗“芯” 与传统机械硬盘相比,固态硬盘最大的特点就是没有磁头、电机、磁盘等复杂的物理结构,而是由纯芯片构成的“电子硬盘”,因此在防震、节能和速度上有着先天的优势。随便拆开一款固态硬盘,都能在PCB板上看到无数颗芯片,而决定固态硬盘性能表现的,则是这些芯片中最为重要的三颗“芯脏”：闪存芯片、主控芯片和缓存芯片。     

与固态硬盘合体！希捷Momentus XT固态混合硬盘

虽然固态硬盘与机械硬盘相比有许多优势,但价格仍难以打动普通消费者。于是不少台式机玩家选择了“折中方案”,即购买小容量的固态硬盘和大容量的机械硬盘一同使用,将操作系统、常用软件等经常读写的文件安装在固态硬盘中以提升性能,而将如电影、游戏等容量大、但不常读写的文件装入机械硬盘。这种方式令台式机获得了显著的性能提升,但对多数笔记本用户来说,这种“混合搭配”的方式几乎不能实现,因为多数笔记本只提供一个硬盘接口,用户不可能增加第二块硬盘。     

SSD都1TB了——KINGMAX 1TB固态硬盘测试

硬盘固态硬盘(SSD)因防震、体积小巧、读写性能出众等特点,赢得了不少硬件发烧友和笔记本电脑用户的青睐。但由于NAND Flash的成本和制造技术的局限,使得它的容量增加缓慢,并且价格一直居高不下。但这次我们带来的是KINGMAX发布不久的,世界首创1TB大容量固态硬盘。     

一种基于硬盘接口的固态盘的设计

本文论述了一种用大容量SRAM芯片构成的固态盘设计方法，并叙述了用此固态盘对早期工控机中硬盘进行代换改造的可行性。     

组合盘节能缓存替换机制

利用组合盘(由固态盘和硬盘构成)进行节能是近年来的一个研究热点.对基于文件粒度的组合盘节能缓存机制开展了研究,利用能量代价模型改编FBR,提出了基于频率和能量的替换算法FEBR;同时,基于滑动窗口机制提出了考虑硬盘寿命的自适应磁盘电源管理机制;为探索现有缓存算法在新硬件结构上的适用性,对过去提出的一系列缓存替换算法在能效、性能、硬盘寿命影响等方面进行了较为全面的比较和评价.通过对收集的4个真实用例数据进行一系列比较实验,得到了如下结论：组合盘节能是可行的,可达70%～80%节能效果;联机替换算法FEBR,FBR以及GDS在所有评价的算法中具有总体最优效果,FEBR略优于FBR和GDS,能耗代价模型可以用于指导算法;所提自适应磁盘管理机制能够较好控制硬盘寿命.     

闪存固态硬盘系统结构与技术

闪存固态硬盘凭借其高性能、低功耗、非易失等特点已经被广泛应用于个人电脑、数据中心和云存储服务等.近10年来,随着制程工艺和微电子技术的发展,闪存固态硬盘的特性发生了显著的变化.首先介绍了闪存存储单元的基本结构和存储原理.然后讨论了闪存固态硬盘的多项控制器关键技术,包括缓存设备、地址转换层、垃圾回收、数据分配、磨损均衡以及纠错码等.这些关键技术将支撑闪存固态硬盘的正常运作.此外,探讨了闪存固态硬盘的并行结构,并分析了闪存固态硬盘并行性利用的限制条件以及最新的并行性优化工作.接着,分析了3D闪存固态硬盘的发展和堆叠式结构,并针对3D固态硬盘的性能和寿命优化工作进行了归纳和分析,提出了现有3D固态硬盘性能和寿命优化工作的不足.最后,总结了当前闪存固态硬盘的现状,并提出可能的未来研究方向.     

【绿色巨无霸】 西部数据Caviar Green WD30EZRX

互联网时代的我们,有太多的数据需要保存。虽然我们对存储设备的性能要求也在随着数据文件的尺寸不断增加,但容量需求的增长却远远高于性能需求。想要追求好的性能,我们可以选择SSD固态硬盘,不过SSD固态硬盘的容量实在是不够看。往大了说,几百个GB而已,而且大容量的SSD固态硬盘价格也高的离谱。想要在SSD固态硬盘上做到性能与容量兼顾,基本上是不现实的。     

HybridPlan: a capacity planning technique for projecting storage requirements in hybrid storage systems

Economic forces, driven by the desire to introduce flash into the high-end storage market without changing existing software-base, have resulted in the emergence of solid-state drives (SSDs), flash packaged in HDD form factors and capable of working with device drivers and I/O buses designed for HDDs. Unlike the use of DRAM for caching or buffering, however, certain idiosyncrasies of NAND Flash-based solid-state drives (SSDs) make their integration into hard disk drive (HDD)-based storage systems nontrivial. Flash memory suffers from limits on its reliability, is an order of magnitude more expensive than the magnetic hard disk drives (HDDs), and can sometimes be as slow as the HDD (due to excessive garbage collection (GC) induced by high intensity of random writes). Given the complementary properties of HDDs and SSDs in terms of cost, performance, and lifetime, the current consensus among several storage experts is to view SSDs not as a replacement for HDD, but rather as a complementary device within the high-performance storage hierarchy. Thus, we design and evaluate such a hybrid storage system with HybridPlan that is an improved capacity planning technique to administrators with the overall goal of operating within cost-budgets. HybridPlan is able to find the most cost-effective hybrid storage configuration with different types of SSDs and HDDs     

PASS: a simple,efficient parallelism-aware solid state drive I/O scheduler

Emerging non-volatile memory technologies, especially flash-based solid state drives （SSDs）, have increasingly been adopted in the storage stack. They provide numerous advantages over traditional mechanically rotating hard disk drives （HDDs） and have a tendency to replace HDDs. Due to the long existence of HDDs as primary building blocks for storage systems, however, much of the system software has been specially designed for HDD and may not be optimal for non-volatile memory media. Therefore, in order to realistically leverage its superior raw performance to the maximum, the existing upper layer software has to be re-evaluated or re-designed. To this end, in this paper, we propose PASS, an optimized I/O scheduler at the Linux block layer to accommodate the changing trend of underlying storage devices toward flash-based SSDs. PASS takes the rich internal parallelism in SSDs into account when dispatching requests to the device driver in order to achieve high performance. Specifically, it parti-tions the logical storage space into fixed-size regions （preferably the component package sizes） as scheduling units. These scheduling units are serviced in a round-robin manner and for every chance that the chosen dispatching unit issues only a batch of either read or write requests to suppress the excessive mutual interference. Additionally, the requests are sorted according to their visiting addresses while waiting in the dispatching queues to exploit high sequential performance of SSD. The experimental results with a variety of workloads have shown that PASS outperforms the four Linux off-the-shelf I/O schedulers by a degree of 3%up to 41%, while at the same time it improves the lifetime significantly, due to reducing the internal write amplification.     

NAND flash memory-based hybrid file system for high I/O performance

As the performance potentials of SSD (Solid State Device) have been recognized, adopting SSD to IT products as HDD replacements is rapidly increasing. Since SSD is organized into multiple flash memory packages, it deploys peculiar device characteristics that do not occur in HDD, such as block-unit erasure overhead. Also, its high cost per capacity is the main obstacle to building a large-scale storage subsystem with only SSDs. An alternative is to build a hybrid storage subsystem where a small portion of SSDs are integrated with HDDs so as to utilize SSD’s performance advantages in a cost-effective way. This study introduces a new form of file system, called N-hybrid (New-Form of hybrid file system), that enables us to support the hybrid device structure combined with both HDD and SSD. Our primary objectives in developing N-hybrid are to provide better I/O bandwidth by exploiting the characteristics of HDD and SSD and to provide a flexible data layout maximizing the usage of tight SSD storage resources. Several experiments were conducted to verify the effectiveness and suitability of N-hybrid.     

Vertical partitioning for flash and HDD database systems

Recent advances in flash memory technology have greatly enhanced the capability of flash memory to address the I/O bottleneck problem. Flash memory has exceptional I/O performance compared to the hard disk drive (HDD). The superiority of flash memory is especially visible when dealing with random read patterns. Even though the cost of flash memory is higher than that of HDD storage, the popularity of flash memory is increasing at such a pace that it is becoming a common addition to the average computer. Recently, flash memory has been made into larger devices called solid state drives (SSDs). Although these devices can offer capacities comparable to HDDs, they are considerably more expensive per byte.Our research aims to increase the I/O performance of database systems by using a small amount of flash memory alongside HDD storage. The system uses a fully vertically partitioned storage structure where each column is stored separately on either the HDD or SSD. Our approach is to assign the columns into the SSD which will benefit the most from the characteristics of flash memory. We prove this problem is NP-complete and propose an optimal dynamic programming solution and a faster greedy heuristic solution.A system simulator has been implemented and experiments show that the overall I/O costs can be greatly reduced using only a limited amount of flash memory. The results show the greedy heuristic solution performed similarly to the more expensive dynamic programming solution for the situations tested.     

Towards trustworthy storage using SSDs with proprietary FTL

In recent years we have seen an increasing deployment of flash-based storage, such as SSD, in mission-critical applications due to its fast read/write speed, small form factor, strong shock resistance, etc.. SSDs use a middle layer called flash translation layer (FTL) to maintain the compatibility with the traditional magnetic-based HDDs. Unlike the traditional HDD where the host OS has the knowledge on where and how to access data, SSD uses FTL to translate and implement all operations. Even worse, FTL, which is considered as one of most important intellectual properties of flash-based storage, is often proprietary. This brings up a serious security concern on design trustworthiness when the manufacturer either accidentally or intentionally implements those operations incorrectly or maliciously. We analyze the possible threats that are brought up by the design trust issues, and propose simple yet effective schemes as countermeasures with overhead evaluation.     

SSD数据结构与算法综述

SSD（Solid State Disk）是一种基于闪存的电可擦除可编程的新型存储器件。与普通硬盘相比,SSD具有访问延迟小、低功耗等优点。但SSD的应用也需要解决写前擦除、损耗平衡等挑战。针对这些挑战,工业界和学术界研究提出了多种复杂的算法和数据结构。对这些研究成果进行了分析和综述,同时探讨了SSD在分层存储中的应用。     

HAT: an efficient buffer management method for flash-based hybrid storage systems

Flash solid-state drives (SSDs) provide much faster access to data compared with traditional hard disk drives (HDDs). The current price and performance of SSD suggest it can be adopted as a data buffer between main memory and HDD, and buffer management policy in such hybrid systems has attracted more and more interest from research community recently. In this paper, we propose a novel approach to manage the buffer in flash-based hybrid storage systems, named hotness aware hit (HAT). HAT exploits a page reference queue to record the access history as well as the status of accessed pages, i.e., hot, warm, and cold. Additionally, the page reference queue is further split into hot and warm regions which correspond to the memory and flash in general. The HAT approach updates the page status and deals with the page migration in the memory hierarchy according to the current page status and hit position in the page reference queue. Compared with the existing hybrid storage approaches, the proposed HAT can manage the memory and flash cache layers more effectively. Our empirical evaluation on benchmark traces demonstrates the superiority of the proposed strategy against the state-of-the-art competitors.     

Thermal analysis of helium-filled enterprise disk drive

Enterprise hard disk drives (HDDs) are widely used in high-end storage systems for data center. One of key performance requirements for enterprise HDDs is data access rate, which demands very high rotational speed (e.g. 15 k rpm or more) to permit fast access time. To reach such high speed, the disk spindle motor draws more power to spin and hence the temperature of HDD enclosure increases due to large windage loss. It has been known, temperature rise is one of the most fundamental factors that affect the reliability of the disk drive. In order to develop high reliable enterprise HDDs, thermal management of enterprise HDDs needs to be optimized to improve heat dissipation. One possible approach is to fill disk drive with helium because of its lower density and higher thermal conductivity. This paper investigates thermal performances of helium-filled enterprise disk drives through FEM simulations with experimental validations. Windage loss and heat convection of the HDD filled with helium and air are analysed. The simulated and measured temperature distributions of one commercial enterprise HDD with helium-filled and helium-air mixture are compared with those of an air-filled one. The results show 41% reduction of temperature rise of HDD enclosure can be achieved by filling with helium in comparison with that of air-filled HDD. It is also projected that in terms of equivalent cooling capability like air-filled HDD at 15 k rpm, helium-filled HDD spindle can spin up to 19 k rpm, which will greatly increase data access rate by 25% for future enterprise applications.     

Human-based evaluation of sound from hard disk drives for noise control

Most recent efforts to reduce the noise produced by hard disk drives (HDDs) have been based on measurable physical quantities such as the sound pressure level. However, given that the purpose of reducing HDD noise is to improve human comfort, it would be better to evaluate noise levels on the basis of human perceptions. To address this issue, we carried out human jury tests on eight HDDs in which people were asked to compare the noise of pairs of HDDs and determine which HDD sounded louder or more annoying. The jury test results were then compared with sound pressure level measurements of the noise produced by the idling HDDs and a discrepancy was found between the human perception of HDD noise and the physical quantities commonly employed for noise control. Psychoacoustic variables such as loudness level showed a better correlation with the jury test results.This research was supported by Samsung Electronics Co., South Korea. The authors gratefully acknowledge the support and encouragement provided by Dr. Young Son and Dr. Yunsik Han.