Dependability Assessment of NAND Flash-memory for Mission-critical Applications

It is a matter of fact that NAND flash memory devices are well established in consumer market. However, it is not true that the same architectures adopted in the consumer market are suitable for mission critical applications like space. In fact, USB flash drives, digital cameras, MP3 players are usually adopted to store "less significant" data which are not changing frequently (e.g., MP3s, pictures, etc.). Therefore, in spite of NAND flash's drawbacks, a modest complexity is usually needed in the logic of commercial flash drives. On the other hand, mission critical applications have different reliability requirements from commercial scenarios. Moreover, they are usually playing in a hostile environment (e.g., the space) which contributes to worsen all the issues. We aim at providing practical valuable guidelines, comparisons and tradeoffs among the huge number of dimensions of fault tolerant methodologies for NAND flash applied to critical environments. We hope that such guidelines will be useful for our ongoing research and for all the interested reader

Dependability Assessment of NAND Flash-memory for Mission-critical Applications

It is a matter of fact that NAND flash memory devices are well established in consumer market. However, it is not true that the same architectures adopted in the consumer market are suitable for mission critical applications like space. In fact, USB flash drives, digital cameras, MP3 players are usually adopted to store "less significant" data which are not changing frequently (e.g., MP3s, pictures, etc.). Therefore, in spite of NAND flash’s drawbacks, a modest complexity is usually needed in the logic of commercial flash drives. On the other hand, mission critical applications have different reliability requirements from commercial scenarios. Moreover, they are usually playing in a hostile environment (e.g., the space) which contributes to worsen all the issues. We aim at providing practical valuable guidelines, comparisons and tradeoffs among the huge number of dimensions of fault tolerant methodologies for NAND flash applied to critical environments. We hope that such guidelines will be useful for our ongoing research and for all the interested readers

초고용량 솔리드 스테이드 드라이브를 위한 신뢰성 향상 및 성능 최적화 기술

학위논문(박사) -- 서울대학교대학원 : 공과대학 컴퓨터공학부, 2021.8. 김지홍.The development of ultra-large NAND flash storage devices (SSDs) is recently made possible by NAND flash memory semiconductor process scaling and multi-leveling techniques, and NAND package technology, which enables continuous increasing of storage capacity by mounting many NAND flash memory dies in an SSD. As the capacity of an SSD increases, the total cost of ownership of the storage system can be reduced very effectively, however due to limitations of ultra-large SSDs in reliability and performance, there exists some obstacles for ultra-large SSDs to be widely adopted. In order to take advantage of an ultra-large SSD, it is necessary to develop new techniques to improve these reliability and performance issues. In this dissertation, we propose various optimization techniques to solve the reliability and performance issues of ultra-large SSDs. In order to overcome the optimization limitations of the existing approaches, our techniques were designed based on various characteristic evaluation results of NAND flash devices and field failure characteristics analysis results of real SSDs. We first propose a low-stress erase technique for the purpose of reducing the characteristic deviation between wordlines (WLs) in a NAND flash block. By reducing the erase stress on weak WLs, it effectively slows down NAND degradation and improves NAND endurance. From the NAND evaluation results, the conditions that can most effectively guard the weak WLs are defined as the gerase mode. In addition, considering the user workload characteristics, we propose a technique to dynamically select the optimal gerase mode that can maximize the lifetime of the SSD. Secondly, we propose an integrated approach that maximizes the efficiency of copyback operations to improve performance while not compromising data reliability. Based on characterization using real 3D TLC flash chips, we propose a novel per-block error propagation model under consecutive copyback operations. Our model significantly increases the number of successive copybacks by exploiting the aging characteristics of NAND blocks. Furthermore, we devise a resource-efficient error management scheme that can handle successive copybacks where pages move around multiple blocks with different reliability. By utilizing proposed copyback operation for internal data movement, SSD performance can be effectively improved without any reliability issues. Finally, we propose a new recovery scheme, called reparo, for a RAID storage system with ultra-large SSDs. Unlike the existing RAID recovery schemes, reparo repairs a failed SSD at the NAND die granularity without replacing it with a new SSD, thus avoiding most of the inter-SSD data copies during a RAID recovery step. When a NAND die of an SSD fails, reparo exploits a multi-core processor of the SSD controller to identify failed LBAs from the failed NAND die and to recover data from the failed LBAs. Furthermore, reparo ensures no negative post-recovery impact on the performance and lifetime of the repaired SSD. In order to evaluate the effectiveness of the proposed techniques, we implemented them in a storage device prototype, an open NAND flash storage device development environment, and a real SSD environment. And their usefulness was verified using various benchmarks and I/O traces collected the from real-world applications. The experiment results show that the reliability and performance of the ultra-large SSD can be effectively improved through the proposed techniques.반도체 공정의 미세화, 다치화 기술에 의해서 지속적으로 용량이 증가하고 있는 단위 낸드 플래쉬 메모리와 하나의 낸드 플래쉬 기반 스토리지 시스템 내에 수 많은 낸드 플래쉬 메모리 다이를 실장할 수 있게하는 낸드 패키지 기술로 인해 하드디스크보다 훨씬 더 큰 초고용량의 낸드 플래쉬 저장장치의 개발을 가능하게 했다. 플래쉬 저장장치의 용량이 증가할 수록 스토리지 시스템의 총 소유비용을 줄이는데 매우 효과적인 장점을 가지고 있으나, 신뢰성 및 성능의 측면에서의 한계로 인해서 초고용량 낸드 플래쉬 저장장치가 널리 사용되는데 있어서 장애물로 작용하고 있다. 초고용량 저장장치의 장점을 활용하기 위해서는 이러한 신뢰성 및 성능을 개선하기 위한 새로운 기법의 개발이 필요하다. 본 논문에서는 초고용량 낸드기반 저장장치(SSD)의 문제점인 성능 및 신뢰성을 개선하기 위한 다양한 최적화 기술을 제안한다. 기존 기법들의 최적화 한계를 극복하기 위해서, 우리의 기술은 실제 낸드 플래쉬 소자에 대한 다양한 특성 평가 결과와 SSD의 현장 불량 특성 분석결과를 기반으로 고안되었다. 이를 통해서 낸드의 플래쉬 특성과 SSD, 그리고 호스트 시스템의 동작 특성을 고려한 성능 및 신뢰성을 향상시키는 최적화 방법론을 제시한다. 첫째로, 본 논문에서는 낸드 플래쉬 불록내의 페이지들간의 특성편차를 줄이기 위해서 동적인 소거 스트레스 경감 기법을 제안한다. 제안된 기법은 낸드 블록의 내구성을 늘리기 위해서 특성이 약한 페이지들에 대해서 더 적은 소거 스트레스가 인가할 수 있도록 낸드 평가 결과로 부터 소거 스트레스 경감 모델을 구축한다. 또한 사용자 워크로드 특성을 고려하여, 소거 스트레스 경감 기법의 효과가 최대화 될 수 있는 최적의 경감 수준을 동적으로 판단할 수 있도록 한다. 이를 통해서 낸드 블록을 열화시키는 주요 원인인 소거 동작을 효율적으로 제어함으로써 저장장치의 수명을 효과적으로 향상시킨다. 둘째로, 본 논문에서는 고용량 SSD에서의 내부 데이터 이동으로 인한 성능 저하문제를 개선하기 위해서 낸드 플래쉬의 제한된 카피백(copyback) 명령을 활용하는 적응형 기법인 rCPB을 제안한다. rCPB은 Copyback 명령의 효율성을 극대화 하면서도 데이터 신뢰성에 문제가 없도록 낸드의 블럭의 노화특성을 반영한 새로운 copyback 오류 전파 모델을 기반으로한다. 이에더해, 신뢰성이 다른 블럭간의 copyback 명령을 활용한 데이터 이동을 문제없이 관리하기 위해서 자원 효율적인 오류 관리 체계를 제안한다. 이를 통해서 신뢰성에 문제를 주지 않는 수준에서 copyback을 최대한 활용하여 내부 데이터 이동을 최적화 함으로써 SSD의 성능향상을 달성할 수 있다. 마지막으로, 본 논문에서는 초고용량 SSD에서 낸드 플래쉬의 다이(die) 불량으로 인한 레이드(redundant array of independent disks, RAID) 리빌드 오버헤드를 최소화 하기위한 새로운 RAID 복구 기법인 reparo를 제안한다. Reparo는 SSD에 대한 교체없이 SSD의 불량 die에 대해서만 복구를 수행함으로써 복구 오버헤드를 최소화한다. 불량이 발생한 die의 데이터만 선별적으로 복구함으로써 복구 과정의 리빌드 트래픽을 최소화하며, SSD 내부의 병렬구조를 활용하여 불량 die 복구 시간을 효과적으로 단축한다. 또한 die 불량으로 인한 물리적 공간감소의 부작용을 최소화 함으로써 복구 이후의 성능 저하 및 수명의 감소 문제가 없도록 한다. 본 논문에서 제안한 기법들은 저장장치 프로토타입 및 공개 낸드 플래쉬 저장장치 개발환경, 그리고 실장 SSD환경에 구현되었으며, 실제 응용 프로그램을 모사한 다양한 벤트마크 및 실제 I/O 트레이스들을 이용하여 그 유용성을 검증하였다. 실험 결과, 제안된 기법들을 통해서 초고용량 SSD의 신뢰성 및 성능을 효과적으로 개선할 수 있음을 확인하였다.I Introduction 1 1.1 Motivation 1 1.2 Dissertation Goals 3 1.3 Contributions 5 1.4 Dissertation Structure 8 II Background 11 2.1 Overview of 3D NAND Flash Memory 11 2.2 Reliability Management in NAND Flash Memory 14 2.3 UL SSD architecture 15 2.4 Related Work 17 2.4.1 NAND endurance optimization by utilizing page characteristics difference 17 2.4.2 Performance optimizations using copyback operation 18 2.4.3 Optimizations for RAID Rebuild 19 2.4.4 Reliability improvement using internal RAID 20 III GuardedErase: Extending SSD Lifetimes by Protecting Weak Wordlines 22 3.1 Reliability Characterization of a 3D NAND Flash Block 22 3.1.1 Large Reliability Variations Among WLs 22 3.1.2 Erase Stress on Flash Reliability 26 3.2 GuardedErase: Design Overview and its Endurance Model 28 3.2.1 Basic Idea 28 3.2.2 Per-WL Low-Stress Erase Mode 31 3.2.3 Per-Block Erase Modes 35 3.3 Design and Implementation of LongFTL 39 3.3.1 Overview 39 3.3.2 Weak WL Detector 40 3.3.3 WAF Monitor 42 3.3.4 GErase Mode Selector 43 3.4 Experimental Results 46 3.4.1 Experimental Settings 46 3.4.2 Lifetime Improvement 47 3.4.3 Performance Overhead 49 3.4.4 Effectiveness of Lowest Erase Relief Ratio 50 IV Improving SSD Performance Using Adaptive Restricted- Copyback Operations 52 4.1 Motivations 52 4.1.1 Data Migration in Modern SSD 52 4.1.2 Need for Block Aging-Aware Copyback 53 4.2 RCPB: Copyback with a Limit 55 4.2.1 Error-Propagation Characteristics 55 4.2.2 RCPB Operation Model 58 4.3 Design and Implementation of rcFTL 59 4.3.1 EPM module 60 4.3.2 Data Migration Mode Selection 64 4.4 Experimental Results 65 4.4.1 Experimental Setup 65 4.4.2 Evaluation Results 66 V Reparo: A Fast RAID Recovery Scheme for Ultra- Large SSDs 70 5.1 SSD Failures: Causes and Characteristics 70 5.1.1 SSD Failure Types 70 5.1.2 SSD Failure Characteristics 72 5.2 Impact of UL SSDs on RAID Reliability 74 5.3 RAID Recovery using Reparo 77 5.3.1 Overview of Reparo 77 5.4 Cooperative Die Recovery 82 5.4.1 Identifier: Parallel Search of Failed LBAs 82 5.4.2 Handler: Per-Core Space Utilization Adjustment 83 5.5 Identifier Acceleration Using P2L Mapping Information 89 5.5.1 Page-level P2L Entrustment to Neighboring Die 90 5.5.2 Block-level P2L Entrustment to Neighboring Die 92 5.5.3 Additional Considerations for P2L Entrustment 94 5.6 Experimental Results 95 5.6.1 Experimental Settings 95 5.6.2 Experimental Results 97 VI Conclusions 109 6.1 Summary 109 6.2 Future Work 111 6.2.1 Optimization with Accurate WAF Prediction 111 6.2.2 Maximizing Copyback Threshold 111 6.2.3 Pre-failure Detection 112박

A (Nearly) Free Lunch:Extending NAND Flash Lifetime by Exploiting Neglected Physical Properties

NAND flash is a key storage technology in modern computing systems. Without it, many devices would probably not exist today or would at least not benefit from as many features. The very large success of this technology motivated massive efforts to scale it down in order to increase its density further. However, NAND flash is currently facing physical limitations that prevent it reaching smaller cell sizes without severely reducing its storage reliability and lifetime. Accordingly, in the present thesis we aim at relieving some constraints from device manufacturing by addressing flash irregularities at a higher level. For example, we acknowledge the fact that process variation plus other factors render some regions of a flash device more sensitive than others. This difference usually leads to sensitive regions exhausting their lifetime early, which then causes the device to become unusable, while the rest of the device is still healthy, yet not exploitable. Consequently, we propose to postpone this exhaustion point with new strategies that require minimal resources to be implemented and effectively extend flash devices lifetime. Sometimes, our strategies involve unconventional methods to access the flash that are not supported by specification document and, therefore, should not be used lightly. Hence, we also present thorough characterization experiments on actual NAND flash chips to validate these methods and model their effect on a flash device. Finally, we evaluate the performance of our methods by implementing a trace-driven flash device simulator and execute a large set of realistic disk traces. Overall, we exploit properties that are either neglected or not understood to propose methods that are nearly free to implement and systematically extend NAND flash lifetime. We are convinced that future NAND flash architectures will regularly bring radical physical changes, which will inevitably come together with a new set of physical properties to investigate and to exploit

Stash in a Flash

Encryption is a useful tool to protect data confidentiality. Yet it is still challenging to hide the very presence of encrypted, secret data from a powerful adversary. This paper presents a new technique to hide data in flash by manipulating the voltage level of pseudo-randomlyselected flash cells to encode two bits (rather than one) in the cell. In this model, we have one “public” bit interpreted using an SLC-style encoding, and extract a private bit using an MLC-style encoding. The locations of cells that encode hidden data is based on a secret key known only to the hiding user. Intuitively, this technique requires that the voltage level in a cell encoding data must be (1) not statistically distinguishable from a cell only storing public data, and (2) the user must be able to reliably read the hidden data from this cell. Our key insight is that there is a wide enough variation in the range of voltage levels in a typical flash device to obscure the presence of fine-grained changes to a small fraction of the cells, and that the variation is wide enough to support reliably re-reading hidden data. We demonstrate that our hidden data and underlying voltage manipulations go undetected by support vector machine based supervised learning which performs similarly to a random guess. The error rates of our scheme are low enough that the data is recoverable months after being stored. Compared to prior work, our technique provides 24x and 50x higher encoding and decoding throughput and doubles the capacity, while being 37x more power efficient