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poised RETHINKING FLASH IN THE DATA CENTER反思闪存在数据中心
——寿浙威 非师
DEPLOYMENT OF FLASH MEMORY DEPENDS ON MAKING THE MOST OF ITS UNIQUE PROPERTIES INSTEAD OF TREATING IT AS A DROP-IN REPLACEMENT FOR EXISTING TECHNOLOGIES.
闪存的未来应用取决于能否充分利用它的特性，而不要把它对待成为一个突然闯入并且来替代现有技术的。
Over the past few years, computer systems of all types have started integrating flash memory. Initially, flash’s small size, low power consumption, and physical durability made it a natural fit for mediaplayers and embedded devices. Lately, flash’s rising density has won it a place in laptops and some desktop machines.
再过去的几年中，各种类型的计算机系统开始集成闪存。一开始，闪存体积小、低功耗以及物理上的持久特性使得它能很好的适合那些媒体播放器、嵌入设备。最近，闪存的高密度集成使它在笔记本电脑和台式电脑上也赢得了一些地位。
Flash is now poised to make deep inroads into the data center. There, flash memory’s high density, low power, and low-cost I/Os per second will drive its adoption and enable its application far beyond simple hard drive replacements. To date, however, many uses of flash have been hamstrung by a fundamental challenge of the technology: Flash is neither magnetic disk nor DRAM. It has its own performance advantages and quirks that system designers must address at several levels to best exploit it.
现在，闪存已经很自然的进入了数据中心。
那些，闪存的高密度、低功耗和输入输出率的低消耗使得它的被采纳度和应用远远超越了简单的对硬盘驱动器的替代。然而，对于数据来说，大量对闪存的使用已经遭到了基础技术的质疑和阻碍：闪存既不是磁盘，也不是动态随机存取存储器。闪存拥有它自身的优点和怪癖，为了更好的利用它，系统设计师必须对它多重编址。
Disk and DRAM replacement
代替磁盘和动态随机存取存储器
So far, most proposed applications for flash in the data center have fallen into two categories.
目前，很多对闪存在数据中心提出的应用可以分为两类。
The first category is disk replacement. Quick access time and low power requirements make flash a compelling replacement for conventional disks, albeit at much lower density and higher cost. Accounting for servers and supporting infrastructure, solid-state disks (SSDs) consume roughly 10 times less energy when idle than disks. They deliver 2.6 times more bandwidth per watt and 3.2 times more bandwidth per dollar, 25 times more I/O operations per second (IOPS) per dollar, and 2,000 times more IOPS per watt .
第一类就是代替磁盘。
快速读取和小功率的要求使得闪存替代密度既低，消耗又高的传统磁盘引人注目，对于服务器的解释和基础设备的支持上，固态硬盘（SSDs）在系统闲置时的消耗时间大约是磁盘的十分之一，它们能传输2.6倍更多的带宽每瓦特，3.2倍更多的带宽每美元，25倍更多的输入输出操作每秒（IOPS）每美元、2000倍更多IOPS每瓦特。
Flash sometimes also serves as a DRAM replacement. Density and (again) energy efficiency let flash compete with DRAM in applications where latency and bandwidth are less important. Flash consumes one-fourth the power of DRAM per byte at one-fifth the price.
闪存有时候也充当动态随机存取存储器的作用。
密度和能量效能使闪存能够在那些潜在因素和带宽不是很重要的应用领域中和DRAM竞争，因为闪存仅仅是DRAM每字节消耗的四分之一，同时价格是DRAM的五分之一。
Flash memory will remain a contender for both roles for the foreseeable future, but additional opportunities and challenges are on the horizon. Technology scaling will continue to increase bit density for another 1 to 2 process generations, which will drive down costs. However, smaller flash cells are less reliable and less durable. In the past two years, lifetime program/erase cycle ratings for high-density flash devices have dropped from 10 thousand to five thousand cycles. Raw bit error rates have increased as well. Understanding how to apply this shifting technology in the changing landscape of datacenter computing requires careful design.
在可预见的未来，闪存会是这两个方面冠军的争夺者。但是，其它的机会和挑战它们都在同一地平线线上。
再用一、二代的过程，缩放技术会继续增加比特位的密度，降低成本。但是，更小的闪存单位会更不可靠，更不稳定。在过去的两年中，对于高密度的闪存设备，编码/擦除的生命周期率已经从10000降到了5000，生成的错误位率也随即增加。
在数据计算不断改变的情况下，想要弄明白如何应用这个狡猾的技术需要细心地设计。
Examples of how blithely applying SSDs to some datacenter applications can result in disappointing performance are easy to find. Our experience with a key-value store designed to hold huge numbers of small key-value pairs elicited nearly worst-case performance from SSDs. The FAWN-KV key-value storage system, developed as a part of Wimpy Nodes (FAWN) project, can handle more than 200 times more inserts per second than the traditional, non-flash-optimized Berkeley DB, on both older flash devices and modern SSDs.
对于如何愉快的把SSDs应用到一些资料处理中心导致令人失望的例子处处可见。我们经历过，从SSDs用一个设计好的关键值来包含大量小的关键值组引出了几乎最糟糕的表现。 FAWN-KV关键值存储系统，作为Wimpy Nodes（FAWN）计划的一部分的发展，可以处理比传统，非闪存优化的Berkeley DB，无论是旧的闪存和现代的SSDs快每秒200倍的输入。
Flash translation layer
闪存翻译层
The flash translation layer (FTL) hides many of flash’s remaining warts. It provides reliability and the abstraction of a uniform block address space. This is necessary since flash cannot do in-place updates, flash cells wear out after between five thousand and one million program/erase cycles, and even read operations can potentially corrupt data. Through heroic engineering and daunting complexity, the FTL masks these problems, but its performance impact can be significant. Intel’s Extreme SSDs have a read latency of 85 ms, but the flash chips the drive uses internally have a read latency of just 25 to 35 ms.
闪存翻译层掩盖了它的一些缺点。它提供了可靠性和统一块空间的抽象，这是必须的，因为闪存不能再适当位置更新，闪存单元会在万次编码/擦除周期后损坏，甚至在读操作时会潜在地产生错误的数据。通过英勇的工程师和令人退缩的复杂性，FTL掩盖了这些问题，但是它所表现的影响是重大的。英特尔最先进的SSDs已经有一个85ms读取反应时间，但是在本地驱动闪存芯片只需要25到35ms读取反应时间。。