Adaptive Read Level Recording for Read Performance Improvement in 3-D NAND Flash

While low-density parity-check code has been adopted in 3-D flash for improving chip reliability, it suffers from severe read latency due to the increasing number of read retries. Recent studies propose read-level recording to mitigate the performance loss from failed read retries. However, existing schemes induce large updating overhead and achieve suboptimal results, making it critical to develop better tradeoffs among storage overhead, process variation, and performance improvement. In this article, we propose AR<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{2}$</tex-math></inline-formula>, an adaptive read-level recording scheme to improve read performance for 3-D NOT AND (NAND) flash. It consists of two designs: AR<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{2}$</tex-math></inline-formula>-win and AR<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{2}$</tex-math></inline-formula>-pre. AR<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{2}$</tex-math></inline-formula>-win records the number of read levels that fit the majority of the last <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$N$</tex-math></inline-formula> reads, which prevents the worst page from dominating the read level recording. AR<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{2}$</tex-math></inline-formula>-pre predicts the number of read levels for the next read based on the recorded one and a simple machine learning model, which prevents using stale recorded levels in large-capacity solid-state drives (SSDs). Our experimental results show that AR<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{2}$</tex-math></inline-formula> significantly improves the read performance for 3-D NAND flash and achieves on average 15% or more read latency reduction over the state-of-the-art.