Multi-level Cell NAND Research Articles

Bilayer LDPC Codes Combined with Perturbed Decoding for MLC NAND Flash Memory.

This paper presents a coding scheme based on bilayer low-density parity-check (LDPC) codes for multi-level cell (MLC) NAND flash memory. The main feature of the proposed scheme is that it exploits the asymmetric properties of an MLC flash channel and stores the extra parity-check bits in the lower page, which are activated only after the decoding failure of the upper page. To further improve the performance of the error correction, a perturbation process based on the genetic algorithm (GA) is incorporated into the decoding process of the proposed coding scheme, which can convert uncorrectable read sequences into error-correctable regions of the corresponding decoding space by introducing GA-trained noises. The perturbation decoding process is particularly efficient at low program-and-erase (P/E) cycle regions. The simulation results suggest that the proposed bilayer LDPC coding scheme can extend the lifetime of MLC NAND flash memory up to 10,000 P/E cycles. The proposed scheme can achieve a better balance between performance and complexity than traditional single LDPC coding schemes. All of these findings indicate that the proposed coding scheme is suitable for practical purposes in MLC NAND flash memory.

A shared page-aware machine learning assisted method for predicting and improving multi-level cell NAND flash memory life expectancy

NAND flash memory is widely used in consumer electronics, personal computers, and enterprise data storage servers. The most prevalent source of flash memory errors is retention errors, which are mainly caused by leakage of charges. It has been observed that inside the chip, some flash memory blocks exhibit greater endurance to the retention errors than others. In an effort to prolong life expectancy, previous studies focus on improving the Raw Bit Error Rate (RBER) metric at page and block levels, without due consideration of the geometry of shared pages. In this paper, the first insight we provide is that groups of shared pages have different RBER values, and should be analyzed separately. We use this insight to construct a machine learning model to predict the blocks which have less endurance, using the characterization data of new unused flash memory chip(s). This is accomplished by extracting location-sensitive and value-sensitive features from the shared pages group, and engineering them into more sophisticated and explainable features. Furthermore, we describe how our proposed prediction model can be used in combination with the existing flash translation layer (FTL) wear-leveling algorithm to increase lifetime. We evaluated the proposed prediction and lifetime improvement method for four different machine learning techniques, among which Support Vector Machine (SVM) achieved superior accuracy up to 85% at a lower computational overhead.

Data Representation Aware of Damage to Extend the Lifetime of MLC NAND Flash Memory

Multilevel cell (MLC) NAND flash memory uses the voltages of the memory cells to represent bits, but high voltages cause much more damage on the cells than low voltages. Free space in MLC can be leveraged to reduce the usage of the high voltages and thus extend the lifetime of MLC. However, limited by the conventional data representation rule that represents bits by the voltage of one single cell, the high voltages are used in a high probability. To fully explore the potential of the free space on reducing the usage of high voltages without changing the total density of MLC, we propose a novel data representation aware of damage, named DREAM. DREAM uses the low voltage combinations of multiple cells instead of the voltage of one single cell to represent bits. It enables to represent the same bits through flexibly replacing the high voltages in some cells with the low voltages in other cells when free space is available. Hence, high voltages which cause more damage are less used and the lifetime of the MLC memory is extended. In addition, complementary techniques are proposed to mitigate performance loss induced by DREAM. Theoretical analysis and simulation results demonstrate the effectiveness and efficiency of DREAM.

Artificial Neural Network Assisted Error Correction for MLC NAND Flash Memory.

The multilevel per cell technology and continued scaling down process technology significantly improves the storage density of NAND flash memory but also brings about a challenge in that data reliability degrades due to the serious noise. To ensure the data reliability, many noise mitigation technologies have been proposed. However, they only mitigate one of the noises of the NAND flash memory channel. In this paper, we consider all the main noises and present a novel neural network-assisted error correction (ANNAEC) scheme to increase the reliability of multi-level cell (MLC) NAND flash memory. To avoid using retention time as an input parameter of the neural network, we propose a relative log-likelihood ratio (LLR) to estimate the actual LLR. Then, we transform the bit detection into a clustering problem and propose to employ a neural network to learn the error characteristics of the NAND flash memory channel. Therefore, the trained neural network has optimized performances of bit error detection. Simulation results show that our proposed scheme can significantly improve the performance of the bit error detection and increase the endurance of NAND flash memory.

A Nonuniform Reference Voltage Optimization Based on Relative-Precision-Loss Ratios in MLC NAND Flash Memory

In Multi-Level-Cell (MLC) NAND flash memory, cell-to-cell interference (CCI) and retention time have become the main noise that degrades the data storage reliability. To mitigate such noise, a relative precision loss (RPL) nonuniform reference voltage sensing strategy is proposed in this paper. First, based on the NAND flash channel model with CCI and retention noise, we simulate the data storage process of MLC NAND flash by Monte Carlo method, and find that the threshold-voltage of each disturbed storage state shows approximately to be Gaussian distributed. Then, by Gaussian approximation, the distribution of threshold voltage can be estimated easily in mathematics with a little loss. Second, we introduce a concept of log-likelihood ratio (LLR)-based RPL ratio to determine the dominating overlap regions, and then propose a new nonuniform reference voltage sensing strategy. This strategy does not only reduce the memory sensing precision (i.e., the number of reference voltages), but also maintains the reliability of the soft information of NAND flash memory channel output for soft decoding. Third, we implement extensive simulations to verify the performance of the new nonuniform sensing strategy. The BER performances of LDPC codes for different sensing strategies are provided to show that the proposed LLR-based RPL-nonuniform sensing strategy can make a good compromise between memory sensing latency and error-correction performance.

Program error mitigation in MLC NAND flash memory with soft decision decoders

Benefiting from strong decoding capabilities, soft-decision decoding has been used to replace hard-decision decoding in various communication systems, and NAND flash memory systems are no exception. However, soft-decision decoding relies heavily on accurate soft information. Owing to the incremental step pulse programming (ISPP), program errors (PEs) in multi-level cell (MLC) NAND flash memory have different characteristics compared to other types of errors, which is very difficult to obtain such accurate soft information. Therefore, the characteristics of the log-likelihood ratio (LLR) of PEs are investigated first in this paper. Accordingly, a PE-aware statistical method is proposed to determine the usage of PE mitigation schemes. In order to reduce the PE estimating workload of the controller, an adaptive blind clipping (ABC) scheme is proposed subsequently to approximate the PEs contaminated LLR with different decoding trials. Finally, simulation results demonstrate that (1) the proposed PE-aware statistical method is effective in practice, and (2) ABC scheme is able to provide satisfactory bit error rate (BER) and frame error rate (FER) performance in a penalty of negligible increasing of decoding latency.

Interleaved Write Scheme for Improving Sequential Write Throughput of Multi-Chip MLC NAND Flash Memory Systems

Mass-storage systems made of multi-level cell (MLC) NAND flash memory are cost effective but suffer from limited sequential write throughput. Although conventional interleaved write scheme helps, the unique characteristic of the MLC that the write time of a least significant bit (LSB) is much shorter than that of a most significant bit (MSB) leaves performance improvement headroom. This article presents an interleaved write scheme for improving the sequential write throughput of the multi-chip MLC NAND flash memory system. It leverages the longer write time of the upper pages to transmit more data by reordering the interleaved write sequence of the MLC chips. The proposed scheme can be directly implemented in the firmware of the flash memory controller without conflicting with existing flash translation layer (FTL) routines. No hardware modification is needed. The design criteria and performance improvements of general MLC NAND flash storage systems are derived for optimizing the system’s performance. Experimental results of an example single-channel 4-chip solid-state drives (SSD) system showed the sequential write throughput was improved by over 11.4%. The proposed interleaved write scheme can be applied to multi-channel storage systems to improve their sequential write throughput as well.

MNFTL

The write constraints of Multi-Level Cell (MLC) NAND flash memory make most of the existing flash translation layer (FTL) schemes inefficient or inapplicable. In this article, we solve several fundamental problems in the design of MLC flash translation layer. The objective is to reduce the garbage collection overhead to reduce the average system response time. We make the key observation that the valid pages copy is the essential garbage collection overhead. Based on this observation, we propose two approaches, namely, concentrated mapping and postponed reclamation, to effectively reduce the valid pages copy. Besides, we propose a progressive garbage collection that can well utilize the system idle time to reclaim more spaces. We conduct a series of experiments on an embedded developing board with a set of benchmarks. The experimental results show that our scheme can achieve a significant reduction in the average system response time compared with the previous work.

An Improved LDPC ECC based on System Level Reprogramming for MLC NAND Flash

With the ever-increasing requirement for higher performance of system and memory with a huge storage capacity, Multi-Level Cell (MLC) flash memory holds a dominant position in flash memory. However, more errors are observed in flash memory as the storage size increases. Thus, various error correction techniques (e.g., Hamming, BCH, LDPC) have been adopted to maintain the reliability of data stored in flash memory. Retention error, which occurs in data that is not accessed for a long period after stored, is the major contributor of flash memory error. The incidence of retention error is highly related to p-e (program-erase) cycle of flash memory. However, in system level, flash memory should be erased before modifying data even if ECC (Error correction code) decoding is completed. This erase operation increases the p-e cycle of flash memory, resulting in high incidence of retention error. This paper introduces a safe in-place reprogramming scheme at system level to optimally correct errors without erasing. Additionally, LDPC decoding scheme uses information about the uncorrected errors to improve the performance of the LDPC decoder. Through an experiment that simulates real NAND flash memory chips, safe reprogramming scheme at system level and proposed LDPC decoding method are explicitly demonstrated. As a result of proposed in-place reprogramming, reliable retention time of data is extended. Also, proposed LDPC decoding along with safe reprogramming scheme reduced the number of average iterations up to 33% while maintaining a robust correction ability.

Pair-Bit Errors Aware LDPC Decoding in MLC NAND Flash Memory

By storing multibit per cell, multilevel cell (MLC) NAND flash memory achieves high storage capacity, but sacrificing data reliability. Error correction codes, such as Bose–Chaudhuri–Hocquenghem (BCH) codes, are widely used to ensure data reliability. However, high raw bit error rates induced by interference noises make BCH codes become insufficient to guarantee data reliability. Low-density parity-check (LDPC) codes are considered as the replacement due to the stronger error correction capability. Nevertheless, directly exploiting LDPC codes introduces a concern about decoding latency because of their iterative decoding in the soft decision process. To develop effective LDPC decoding algorithms, it is necessary to have a more profound understanding on flash failure patterns. This paper first observes the pair-bit errors (PBEs) characteristic of MLC NAND flash memory on a real field-programmable gate array testing platform, then proposes a PBE-aware LDPC (PAL) decoding scheme-based upon this observation, in which PBE provides the promotion information for LDPC decoding to reduce decoding latency. Simulation results show that the decoding latency can be reduced by up to 54%, compared with the conventional LDPC codes.

Interpage-Based Endurance-Enhancing Lower State Encoding for MLC and TLC Flash Memory Storages

During the past decade, the endurance of NAND flash memory has severely deteriorated. The maximum number of program and erase cycles has fallen significantly with emerging of multilevel cell (MLC) and triple-level cell (TLC) technology, and scaling down of the cell size. Wear leveling is a general solution used to alleviate this issue; it enables cells to wear down evenly but it cannot actually mitigate the wearing of the cells. Accordingly, techniques are required to minimize the actual cell degradation. This paper proposes <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">endurance-enhancing lower state encoding</i> . The key insight leveraged by the proposed technique is the data pattern-related characteristic of MLC and TLC NAND flash memories, in which the lower the state of the cells, the lower the occurrence of wear out. Thus, our proposed scheme encodes input data to make the cell state as low as possible in consideration of interpage relation. As a result, the wear out of the memory cells can be minimized and their lifetime is improved by 62.7% in a file type and 43.0% in MySQL. Experimental results indicate that our scheme shows better lifetime improvement than other schemes in most cases.

3-D NAND Flash Value-Aware SSD: Error-Tolerant SSD Without ECCs for Image Recognition

This paper proposes Value-Aware solid-state drive (SSD) with fast access speed and low power consumption by eliminating error-correcting code (ECC). Value-Aware SSD utilizes the error tolerance of image recognition application using a deep neural network (DNN) to enhance reliability. In a previous paper, which proposes Value-Aware SSD, fast ECC decoder is implemented and SSD is evaluated with the 32-bit floating-point data format. On the other hand, in this paper, the proposed Value-Aware SSD is analyzed with 32-bit and 8-bit fixed-point data format and achieves the higher reliability even without ECC by newly proposed two techniques, Critical Bit Error Reduction (CBER) and Middle & Lower Page Error Reduction (M&L-PER). CBER and M&L-PER are proposed for 32-bit and 8-bit data format of application, respectively. These techniques modulate the threshold voltage ( $V_{\mathrm {TH}}$ ) distribution of memory cells by recognizing the importance of each stored bit. By proposed CBER, as much as 15% bit error rate (BER) of NAND flash is allowed while the application provides high image recognition accuracy. Even if bit precision is truncated to 8 bit, 3.9% BER is accepted by M&L-PER. The fast read access and low power consumption are realized because ECC is not required. Finally, this paper analyzes the Value-Aware techniques with 3-D multi-level cell (MLC) NAND flash to compare the effects for 3D-MLC and triple-level cell (TLC) NAND flash.

Protograph QC-LDPC and Rate-Adaptive Polar Codes Design for MLC NAND Flash Memories

A protograph-based quasi-cyclic (QC) low-density parity-check (LDPC) code for multi-level cell (MLC) NAND flash memories is proposed in this paper. In this design approach, the quantized voltage signals are measured for soft decoding because the exact voltage values are unavailable. Flash memory channels are asymmetric, and therefore, not optimal for existing LDPC codes optimized for symmetric additive white Gaussian noise (AWGN)-like channels. Mutual information (MI) between the input and output of flash memory is used to model the quantized log-likelihood ratio (LLR) messages. The base matrix of the method is constructed according to the degree sequences optimized by the modified extrinsic information transfer (EXIT) chart method for flash memories. The designed protograph-based codes have a low-complexity QC encoder structure with a readily parallelizable decoder structure. In addition, rate-adaptive polar code design based on Bhattacharyya parameters of the memory cell bits is proposed to further improve the storage efficiency of the MLC NAND flash memories. The code design takes advantage of the inherent characteristics of MLC flash memory channel to iteratively calculate the Bhattacharyya parameter of each memory cell bit, where the punctured positions are selected by choosing the bits with higher Bhattacharyya parameters to construct rate-adaptive polar codes. The simulation results confirm the benefits of the proposed coding schemes.

Polar-coded forward error correction for MLC NAND flash memory

With the ever-growing storage density, high-speed, and low-cost data access, flash memory has inevitably become popular. Multi-level cell (MLC) NAND flash memory, which can well balance the data density and memory stability, has occupied the largest market share of flash memory. With the aggressive memory scaling, however, the reliability decays sharply owing to multiple interferences. Therefore, the control system should be embedded with a suitable error correction code (ECC) to guarantee the data integrity and accuracy.We proposed the pre-check scheme which is a multi-strategy polar code scheme to strike a balance between reasonable frame error rate (FER) and decoding latency. Three decoders namely binary-input, quantized-soft, and pure-soft decoders are embedded in this scheme. Since the calculation of soft log-likelihood ratio (LLR) inputs needs multiple sensing operations and optional quantization boundaries, a $2$-bit quantized hard-decision decoder is proposed to outperform the hard-decoded LDPC bit-flipping decoder with fewer sensing operations. We notice that polar codes have much lower computational complexity compared with LDPC codes. The stepwise maximum mutual information (SMMI) scheme is also proposed to obtain overlapped boundaries without exhausting search. The mapping scheme using Gray code is employed and proved to achieve better raw error performance compared with other alternatives. Hardware architectures are also given in this paper.

Low-complexity detection and decoding scheme for LDPC-coded MLC NAND flash memory

With the development of manufacture technology, the multi-level cell (MLC) technique dramatically increases the storage density of NAND flash memory. As the result, cell-to-cell interference (CCI) becomes more serious and hence causes an increase in the raw bit error rate of data stored in the cells. Recently, low-density parity-check (LDPC) codes have appeared to be a promising solution to combat the interference of MLC NAND flash memory. However, the decoding complexity of the sum-product algorithm (SPA) is extremely high. In this paper, to improve the accuracy of the log likelihood ratio (LLR) information of each bit in each NAND flash memory cell, we adopt a non-uniform detection (N-UD) which uses the average maximum mutual information to determine the value of the soft-decision reference voltages. Furthermore, with an aim to reduce the decoding complexity and improve the decoding performance, we propose a modified soft reliability-based iterative majority-logic decoding (MSRBI-MLGD) algorithm, which uses a non-uniform quantizer based on power function to decode LDPC codes. Simulation results show that our design can offer a desirable trade-off between the performance and complexity for high-column-weight LDPC-coded MLC NAND flash memory.

A joint-LDPC decoding scheme based on retention error characteristics for MLC NAND flash memory

With the aggressive scaling and the Multi-Level Cell (MLC) technology, the reliability of NAND flash degrades seriously. Therefore, the traditional error correction schemes are becoming less and less effective. Low-Density Parity-Check (LDPC) code and soft-decision memory sensing will be more suitable in the next generation Error Correction Code (ECC) technologies for MLC NAND flash memory. However, LDPC ECC schemes have their own bottleneck due to the unacceptably long read latency during the decoding scheme. The read latency primarily includes two aspects: repeated read cycles and iterative belief-propagation decoding. Motivated by the precise probability information can improve the performance of LDPC decoding, we propose a novel Joint-LDPC (J-LDPC) scheme to reduce the decoding latency of LDPC ECC. The key idea of the proposed J-LDPC scheme is to introduce a new calculating method of probability information in a pair of shared flash pages, according to the retention error characteristics of MLC NAND flash rather than several reading operations. Through a large amount of experiments from our own hardware-software co-designed experimental platform, compared with the traditional soft-decision LDPC scheme, the proposed J-LDPC scheme can reduce the average iterations of LDPC decoder by 50% and enhance the error correction strength by 58%.

GCMix

In multi-level cell (MLC) NAND flash memory, two logical pages are overlapped on a single physical page. Even after a logical page is programmed, the data can be corrupted if the programming of the coexisting logical page is interrupted. This phenomenon is called paired page interference. This article proposes a novel software technique to deal with the paired page interference without any additional hardware or extra page write. The proposed technique utilizes valid pages in the victim block during garbage collection (GC) as the backup against the interference, and pairs them with incoming pages written by the host. This approach eliminates undesirable page copy to backup pages against the interference. However, such a strategy has an adverse effect on the hot/cold separation policy, which is essential to improve the efficiency of GC. To limit the downside, we devise a metric to estimate the benefit of GCMix on-the-fly so that GCMix can be adaptively utilized only when the benefit outweighs the overhead. Evaluations using synthetic and real workloads show GCMix can effectively deal with the paired page interference, reducing the write amplification factor by up to 17.5%compared to the traditional technique, while providing comparable I/O performance.

A Block-Level Log-Block Management Scheme for MLC NAND Flash Memory Storage Systems

NAND flash memory is the major storage media for both mobile storage cards and enterprise Solid-State Drives (SSDs). Log-block-based Flash Translation Layer (FTL) schemes have been widely used to manage NAND flash memory storage systems in industry. In log-block-based FTLs, a few physical blocks called log blocks are used to hold all page updates from a large amount of data blocks. Frequent page updates in log blocks introduce big overhead so log blocks become the system bottleneck. To address this problem, this paper presents BLog , a block-level log-block management scheme for MLC NAND flash memory storage system. In BLog, with block-level management, the update pages of a data block can be collected together and put into the same log block as much as possible; therefore, we can effectively reduce the associativities of log blocks so as to reduce the garbage collection overhead. We also propose a novel partial merge operation strategy called reduced-order merge by which we can effectively postpone the garbage collection of log blocks so as to maximally utilize valid pages and reduce unnecessary erase operations in log blocks. Based on BLog, we design an FTL called BLogFTL for Multi-Level Cell (MLC) NAND flash. We conduct a set of experiments on a real hardware platform. Both representative FTL schemes and the proposed BLogFTL have been implemented in the hardware evaluation board. The experimental results show that our scheme can effectively reduce the garbage collection operations and reduce the system response time compared to the previous log-block-based FTLs for MLC NAND flash.

LDPC coding scheme for improving the reliability of multi-level-cell NAND flash memory in radiation environments

Utilizing commercial off-the-shelf (COTS) components in satellites has received much attention due to the low cost. However, commercial memories suffer severe reliability problems in radiation environments. This paper studies the low-density parity-check (LDPC) coding scheme for improving the reliability of multi-level-cell (MLC) NAND Flash memory in radiation environments. Firstly, based on existing physical experiment works, we introduce a new error model for heavy-ion irradiations; secondly, we explore the optimization of writing voltage allocation to maximize the capacity of the storage channel; thirdly, we design the degree distribution of LDPC codes that is specially suitable for the proposed model; finally, we propose a joint detection-decoding scheme based on LDPC codes, which estimates the storage channel state and executes an adaptive log-likelihood ratio (LLR) calculation to achieve better performance. Simulation results show that, compared with the conventional LDPC coding scheme, the proposed scheme may almost double the lifetime of the MLC NAND Flash memory in radiation environments.

Exploiting Write Heterogeneity of Morphable MLC/SLC SSDs in Datacenters with Service-Level Objectives

Given the needs of data-intensive web services and cloud computing applications, storage centers play an important role in serving the demanded data access while jointly considering low cost, qualified performance, and good scalability. To manage peak workloads with performance requirements for read/write latencies, overprovisioning more storage nodes is common but also increases total cost as well as power consumption. Recently, due to the growing capacity and dropping price, NAND-flash-based Solid-State Drives (SSDs) have become an attractive storage solution in datacenters. In this work, we exploit the write heterogeneity in Multi-Level-Cell (MLC) NAND flash memory to meet Service-Level Objectives (SLOs) of applications and to avoid storage overprovision. In MLC NAND flash memory, a memory cell can be programmed as a Single-Level Cell (SLC) or a multi-level cell at runtime, and SLC writes take shorter latency with the cost of larger consumed capacity. The proposed SLO-aware morphable SSD design seeks to meet the SLO requirement by deciding the write mode of each write request while minimizing the number of SLC writes. Experimental results show that the proposed design meets the SLO requirement for all of the tested I/O traces with less than 2.8 percent extra erase counts in average, while conventional MLC SSDs require up to 2.375 times storage overprovision to meet the SLO requirement.