基于闪存的固态驱动器中的错误表征、减轻和恢复
Error Characterization, Mitigation, and Recovery in Flash Memory Based Solid-State Drives
摘要：
NAND闪存在今天的日常生活中无处不在，因为其容量在过去几十年中不断增加并且成本不断降低。这种积极的增长是两个关键趋势的结果：（1）有效的工艺技术扩展，以及（2）多级（例如，MLC、TLC）单元数据编码。不幸的是，存储在闪存中的原始数据的可靠性也继续变得更加难以确保，因为这两个趋势导致（1）在闪存单元（浮置栅极）中更少的电子来表示数据，以及（2）更大的单元间干扰和扰动效应。如果不采取缓解措施，可靠性恶化会缩短NAND闪存的寿命。因此，固态驱动器（SSD）中的闪存控制器变得更加复杂：它们结合了许多有效的技术来确保正确解释存储在闪存单元中的噪声数据。在这篇文章中，我们回顾了SSD错误表征，缓解和数据恢复技术的可靠性和寿命改善的最新进展。我们提供了严格的实验数据，从国家的最先进的MLC和TLC NAND闪存设备上的各种类型的闪存错误，以激励这种技术的需要。基于对实验表征的理解，我们描述了几种缓解和恢复技术，包括（1）小区间干扰缓解，（2）最佳多级小区感测，（3）使用最先进的算法和方法进行纠错，以及（4）纠错失败时的数据恢复。我们量化了这些技术提供的可靠性提高。展望未来，我们简要讨论了闪存和这些技术如何发展到未来。
英文摘要：
NAND flash memory is ubiquitous in everyday life today because its capacity has continuously increased and cost has continuously decreased over decades. This positive growth is a result of two key trends: (1) effective process technology scaling, and (2) multi-level (e.g., MLC, TLC) cell data coding. Unfortunately, the reliability of raw data stored in flash memory has also continued to become more difficult to ensure, because these two trends lead to (1) fewer electrons in the flash memory cell (floating gate) to represent the data and (2) larger cell-to-cell interference and disturbance effects. Without mitigation, worsening reliability can reduce the lifetime of NAND flash memory. As a result, flash memory controllers in solid-state drives (SSDs) have become much more sophisticated: they incorporate many effective techniques to ensure the correct interpretation of noisy data stored in flash memory cells. In this article, we review recent advances in SSD error characterization, mitigation, and data recovery techniques for reliability and lifetime improvement. We provide rigorous experimental data from state-of-the-art MLC and TLC NAND flash devices on various types of flash memory errors, to motivate the need for such techniques. Based on the understanding developed by the experimental characterization, we describe several mitigation and recovery techniques, including (1) cell-to-cell interference mitigation, (2) optimal multi-level cell sensing, (3) error correction using state-of-the-art algorithms and methods, and (4) data recovery when error correction fails. We quantify the reliability improvement provided by each of these techniques. Looking forward, we briefly discuss how flash memory and these techniques could evolve into the future.
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