DIR: Dynamic Request Interleaving for Improving the Read Performance of Aged SSDs
DIR: Dynamic Request Interleaving for Improving the Read Performance of Aged SSDs
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DOI：
10.1109/nvmsa.2019.8863520
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发表时间：
2019-08
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影响因子：
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通讯作者：
Shiqiang Nie;Youtao Zhang;Weiguo Wu;Chi Zhang;Jun Yang
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文献类型：
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作者：
Shiqiang Nie;Youtao Zhang;Weiguo Wu;Chi Zhang;Jun Yang
TLC（三级单元）NAND 闪存越来越多地用于构建现代计算机系统的 SSD（固态硬盘）。 TLC NAND闪存虽然有效提高了存储密度，但也面临着严重的可靠性问题，特别是使用不同位存储的页面表现出不同的BER（误码率）。集成强大的 LDPC（低密度奇偶校验码）有助于提高可靠性，但会因多次读取重试而遭受较长且成比例的读取延迟。在本文中，我们提出了 DIR，这是一种提高基于 TLC NAND 闪存的 SSD 性能的新策略，特别是具有大 BER 的老化 SSD。 DIR 利用了这样的观察结果：在不考虑排队时间的情况下，I/O 请求的延迟是通过访问最慢的设备页面（即具有最高 BER 的页面）来确定的。通过对具有高局部性的连续逻辑页面进行分组，并将其编码数据交错在具有不同 RBER 的三种不同类型的设备页面中，DIR 有效地减少了 LDPC 的读取重试次数。我们的实验结果表明，在老化的 SSD 中采用 DIR 可以利用 I/O 请求的近 75% 的局部性，并且与传统的老化 SSD 相比，平均读取延迟减少 36%。
TLC (Triple-Level Cell) NAND flash is increasingly adopted to build SSDs (Solid-State Drives) for modern computer systems. While TLC NAND flash effectively improves storage density, it faces severe reliability issues, in particular, the pages stored using different bits exhibit different BERs (bit error rates). Integrating strong LDPC (Low-Density Parity-Check code) helps to improve reliability but suffers from long and proportional read latency due to multiple read retries. In this paper, we propose DIR, a novel strategy for improving the performance of TLC NAND flash-based SSDs, in particular, the aged ones with large BERs. DIR exploits the observation that the latency of an I/O request is determined, without considering the queuing time, by the access of the slowest device page, i.e., the page that has the highest BER. By grouping consecutive logical pages that have high locality, and interleaving their encoded data in three different types of device pages that have different RBERs, DIR effectively reduces the number of read retries for LDPC. Our experimental results show that adopting DIR in aged SSDs exploits nearly 75% locality from I/O requests, and, on average, reduces 36% read latency over conventional aged SSDs.