Onyx: A Prototype Phase Change Memory Storage Array

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摘要：

We describe a prototype high-performance solid-state drive based on first-generation phase-change memory (PCM) devices called Onyx. Onyx has a capacity of 10 GB and connects to the host system via PCIe. We describe the internal architecture of Onyx including the PCM memory modules we constructed and the FPGA-based controller that manages them. Onyx can perform a 4 KB random read in 38 μs and sustain 191K 4 KB read IO operations per second. A 4 KB write requires 179 μs. We describe our experience tuning the Onyx system to reduce the cost of wear-leveling and increase performance. We find that Onyx out-performs a state-of-the-art flash-based SSD for small writes (< 2 KB) by between 72 and 120% and for reads of all sizes. In addition, Onyx incurs 20-51% less CPU overhead per IOP for small requests. Combined, our results demonstrate that even first-generation PCM SSDs can out-perform flash-based arrays for the irregular (and frequently read-dominated) access patterns that define many of today's "killer" storage applications. Next generation PCM devices will widen the performance gap further and set the stage for PCM becoming a serious flash competitor in many applications.

该文描述了基于第一代PCM设备的高性能SSD原型Onyx。该原型通过PCIE连接，具有10GB容量。给出了其内部结构，包括PCM模块和基于FPGA的控制器。

性能：一次4 KB随机读  38μs；191K次 4 KB读操作 每秒；一次4 KB写 179 μs。

实验描述了如何降低wear-leveling开销，增加性能；

发现：该原型性能超过当前的基于flash的SSD——小写（<2KB）性能：72%-120%，各种大小的读操作性能。同时，对于小请求发生小于20-25%CPU开销。

在非规则访问模式下，第一代的PCM SSD性能仍超过基于Flash的阵列。

结论：下一代PCM继续扩大该性能差异，成为flash的取代者。

注1：PCM

相变存储器（PCM）是一种非易失存储设备，它利用材料的可逆转的相变来存储信息。同一物质可以在诸如固体、液体、气体、冷凝物和等离子体等状态下存在，这些状态都称为相。相变存储器便是利用特殊材料在不同相间的电阻差异进行工作的。

该论文描述了PCM存储设计的特点，相对于flash SSD。

参考：http://www.dzsc.com/data/html/2009-11-13/80113.html

http://www.docin.com/p-87626039.html

注2：Wear-leveling

关于SSD的寿命，MLC芯片的参数是写一万次，虽然看上去有点小，不过主控芯片会使用一种叫Wear Leveling(磨损平衡)的机制，就是把写分散在每个块上，来延长寿命。Wear leveling可保证平均使用FLASH闪存内的每一个区域的一种软件技术。使用该技术的好处在于：可以让FLASH闪存芯片的寿命更长，出错率更低。具体有动态模式和静态模式。