Redundant Array Of Independent Disks Research Articles

Improving 3D NAND Flash Memory Lifetime by Tolerating Early Retention Loss and Process Variation

Compared to planar (i.e., two-dimensional) NAND flash memory, 3D NAND flash memory uses a new flash cell design, and vertically stacks dozens of silicon layers in a single chip. This allows 3D NAND flash memory to increase storage density using a much less aggressive manufacturing process technology than planar NAND flash memory. The circuit-level and structural changes in 3D NAND flash memory significantly alter how different error sources affect the reliability of the memory. In this paper, through experimental characterization of real, state-of-the-art 3D NAND flash memory chips, we find that 3D NAND flash memory exhibits three new error sources that were not previously observed in planar NAND flash memory: (1) layer-to-layer process variation, a new phenomenon specific to the 3D nature of the device, where the average error rate of each 3D-stacked layer in a chip is significantly different; (2) early retention loss, a new phenomenon where the number of errors due to charge leakage increases quickly within several hours after programming; and (3) retention interference, a new phenomenon where the rate at which charge leaks from a flash cell is dependent on the data value stored in the neighboring cell. Based on our experimental results, we develop new analytical models of layer-to-layer process variation and retention loss in 3D NAND flash memory. Motivated by our new findings and models, we develop four new techniques to mitigate process variation and early retention loss in 3D NAND flash memory. Our first technique, Layer Variation Aware Reading (LaVAR), reduces the effect of layer-to-layer process variation by fine-tuning the read reference voltage separately for each layer. Our second technique, Layer-Interleaved Redundant Array of Independent Disks (LI-RAID), uses information about layer-to-layer process variation to intelligently group pages under the RAID error recovery technique in a manner that reduces the likelihood that the recovery of a group fails significantly earlier than the recovery of other groups. Our third technique, Retention Model Aware Reading (ReMAR), reduces retention errors in 3D NAND flash memory by tracking the retention time of the data using our new retention model and adapting the read reference voltage to data age. Our fourth technique, Retention Interference Aware Neighbor-Cell Assisted Correction (ReNAC), adapts the read reference voltage to the amount of retention interference a page has experienced, in order to re-read the data after a read operation fails. These four techniques are complementary, and can be combined together to significantly improve flash memory reliability. Compared to a state-of-the-art baseline, our techniques, when combined, improve flash memory lifetime by 1.85×. Alternatively, if a NAND flash vendor wants to keep the lifetime of the 3D NAND flash memory device constant, our techniques reduce the storage overhead required to hold error correction information by 78.9%.

A Unified Form of EVENODD and RDP Codes and Their Efficient Decoding

Array codes are used widely in data storage systems such as redundant array of independent disks. The row-diagonal parity (RDP) codes and EVENODD codes are two popular double-parity array codes. The increasing capacity of hard disks demands better fault tolerance by using array codes with three or more parity disks. Although many extensions of RDP and EVENODD codes have been proposed, their main drawback is high decoding complexity. In this paper, we propose a unified form of RDP and EVENODD codes under which RDP codes can be treated as shortened EVENODD codes. Moreover, an efficient decoding algorithm based on an LU factorization of a Vandermonde matrix is proposed. The LU decoding method is applicable to all the erasure patterns of RDP and EVENODD codes with three parity columns. It is also applicable to the erasure decoding of RDP and EVENODD codes with more than three parity columns when the number of continuous surviving parity columns is no less than the number of erased information columns and the first parity column has not failed. The proposed efficient decoding algorithm is also applicable to other Vandermonde array codes, with less decoding complexity than that of the existing method.

Modeling Impact of Human Errors on the Data Unavailability and Data Loss of Storage Systems

Data storage systems (DSSs) and their availability play a crucial role in contemporary datacenters. Despite using mechanisms such as automatic failover in datacenters, the role of human agents and consequently their destructive errors is inevitable. Due to very large number of disk drives used in exascale datacenters and their high failure rates, the disk subsystem in storage systems has become a major source of data unavailability (DU) and data loss (DL) initiated by human errors. In this paper, we investigate the effect of incorrect disk replacement service (IDRS) on the availability and reliability of DSSs. To this end, we analyze the consequences of IDRS in a disk array, and conduct Monte Carlo simulations to evaluate DU and DL during mission time. The proposed modeling framework can cope with different storage array configurations and data object survivability , representing the effect of system-level redundancies such as remote backups and mirrors. In the proposed framework, the model parameters are obtained from industrial and scientific reports alongside field data, which have been extracted from a datacenter operating with 70 storage racks. The results show that ignoring the impact of IDRS leads to unavailability underestimation by up to three orders of magnitude. Moreover, our study suggests that by considering the effect of human errors, the conventional beliefs about the dependability of different redundant array of independent disks (RAID) mechanisms should be revised. The results show that $\text{RAID}1$ can result in lower availability compared to $\text{RAID}5$ in the presence of human errors. The results also show that employing automatic fail-over policy (using hot spare disks) can reduce the drastic impacts of human errors by two orders of magnitude.

A Mirroring-Assisted Channel-RAID5 SSD for Mobile Applications

Simply applying an existing redundant array of independent disks (RAID) technique to enhance data reliability within a single solid-state drive for safety-critical mobile applications significantly degrades performance. In this article, we first propose a new RAID5 architecture called channel-RAID5 with mirroring (CR5M) to alleviate the performance degradation problem. Next, an associated data reconstruction strategy called mirroring-assisted channel-level reconstruction (MCR) is developed to further shrink the window of vulnerability. Experimental results demonstrate that compared with channel-RAID5 (CR5), CR5M improves performance up to 40.2%. Compared with disk-oriented reconstruction, a traditional data reconstruction scheme, MCR on average improves data recovery speed by 7.5% while delivering a similar performance during reconstruction.

FPGA-Based Data Storage System on NAND Flash Memory in RAID 6 Architecture for In-Line Pipeline Inspection Gauges

In this manuscript, we present a redundant data storage system based on NAND flash memory chips for in-line Pipeline Inspection Gauges (PIGs). The system is the next step for a technique that reduces data from 1,024 to 37 bytes by 80 transducers used for straight-beam ultrasonic inspection. Each inspection is costly, because PIGs check pipelines up to 100 Km, collecting data every 3 mm and reaching speeds of 2 m/s. These conditions require that the storage system must be redundant, and able to maintain a minimum data flow, thus avoiding bottlenecks. To achieve this, we analyzed the variables that influence the inspection process in real-time, and we structured our Flash Translation Layer (FTL) to eliminate the latencies generated by the computation of the Error Correcting Codes (ECC) and redundancy bytes. Our controller computes the ECC and redundancy bytes while it transfers the information to the cache register of the selected die in the memory chips. At the hardware level, we interleaved 8 NAND flash chips in a Redundant Array of Independent Disks (RAID) type-6 architecture. We tested the storage system considering the incorrect response of up to 2 chips and ensuring a throughput up to 7.28 MB/s. Finally, we expanded the analysis of the data flow, whereby this system is profitable for different pipeline diameters or compression techniques.

Improving 3D NAND Flash Memory Lifetime by Tolerating Early Retention Loss and Process Variation

Compared to planar NAND flash memory, 3D NAND flash memory uses a new flash cell design, and vertically stacks dozens of silicon layers in a single chip. This allows 3D NAND flash memory to increase storage density using a much less aggressive manufacturing process technology than planar NAND. The circuit-level and structural changes in 3D NAND flash memory significantly alter how different error sources affect the reliability of the memory. Our goal is to (1)~identify and understand these new error characteristics of 3D NAND flash memory, and (2)~develop new techniques to mitigate prevailing 3D NAND flash errors. \chIIIn this paper, we perform a rigorous experimental characterization of real, state-of-the-art 3D NAND flash memory chips, and identify three new error characteristics that were not previously observed in planar NAND flash memory, but are fundamental to the new architecture of 3D NAND flash memory. \beginenumerate [leftmargin=13pt] ıtem 3D NAND flash memory exhibits layer-to-layer process variation, a new phenomenon specific to the 3D nature of the device, where the average error rate of each 3D-stacked layer in a chip is significantly different. We are the first to provide detailed experimental characterization results of layer-to-layer process variation in real flash devices in open literature. Our results show that the raw bit error rate in the middle layer can be 6× the error rate in the top layer. ıtem 3D NAND flash memory experiences \emphearly retention loss, a new phenomenon where the number of errors due to charge leakage increases quickly within several hours after programming, but then increases at a much slower rate. We are the first to perform an extended-duration observation of early retention loss over the course of 24~days. Our results show that the retention error rate in a 3D NAND flash memory block quickly increases by an order of magnitude within $\sim$3 hours after programming. ıtem 3D NAND flash memory experiences retention interference, a new phenomenon where the rate at which charge leaks from a flash cell is dependent on the amount of charge stored in neighboring flash cells. Our results show that charge leaks at a lower rate (i.e., the retention loss speed is slower) when the neighboring cell is in a state that holds more charge (i.e., a higher-voltage state). \endenumerate Our experimental observations indicate that we must revisit the error models and error mitigation mechanisms devised for planar NAND flash, as they are no longer accurate for 3D NAND flash behavior. To this end, we develop \emphnew analytical model\chIs of (1)~the layer-to-layer process variation in 3D NAND flash memory, and (2)~retention loss in 3D NAND flash memory. Our models estimate the raw bit error rate (RBER), threshold voltage distribution, and the \emphoptimal read reference voltage (i.e., the voltage at which RBER is minimized when applied during a read operation) for each flash page. Both models are useful for developing techniques to mitigate raw bit errors in 3D NAND flash memory. Motivated by our new findings and models, we develop four new techniques to mitigate process variation and early retention loss in 3D NAND flash memory. Our first technique, LaVAR, reduces process variation by fine-tuning the read reference voltage independently for each layer. Our second technique, LI-RAID, improves reliability by changing how pages are grouped under the RAID (Redundant Array of Independent Disks) error recovery technique, using information about layer-to-layer process variation to reduce the likelihood that the RAID recovery of a group could fail significantly earlier during the flash lifetime than recovery of other groups. Our third technique, ReMAR, reduces retention errors in 3D NAND flash memory by tracking the retention age of the data using our retention model and adapting the read reference voltage to data age. Our fourth technique, ReNAC, adapts the read reference voltage to the amount of retention interference to re-read the data after a read operation fails. These four techniques are complementary, and can be combined together to significantly improve flash memory reliability. Compared to a state-of-the-art baseline, our techniques, when combined, improve flash memory lifetime by 1.85×. Alternatively, if a NAND flash manufacturer wants to keep the lifetime of the 3D NAND flash memory device constant, our techniques reduce the storage overhead required to hold error correction information by 78.9%. For more information on our new experimental characterization of modern 3D NAND flash memory chips and our proposed models and techniques, please refer to the full version of our paper~\citeluo.pomacs18.

Fair bandwidth allocating and strip-aware prefetching for concurrent read streams and striped RAIDs in distributed file systems

With a striped RAID (Redundant Array of Independent Disks) which consists of multiple disks and spreads data across them in parallel, distributed file systems (DFSs) easily enhance the performance of a single read stream (i.e., a series of sequential reads by a process). However, most existing DFSs suffer from performance degradation in concurrent read streams (i.e., multiple series of sequential reads by concurrent processes). Furthermore, research on the performance of concurrent ones for a striped RAID in DFSs has been rarely reported so far. In this paper, we define the problems that degrade it at different configurations of striped RAIDs, and resolve them by proposing the following two methods: (1) a fair allocating of network bandwidth for concurrent read streams and (2) a strip-aware prefetching for each individual read stream. We show that our proposal outperforms all the existing DFSs by at least two times for all kinds and configurations of striped RAIDs. Furthermore, the performance gap between our proposal and the existing DFSs becomes wider according to the increasing number of striped disks.

Modeling the Endurance Reliability of Intradisk RAID Solutions for Mid-1X TLC NAND Flash Solid-State Drives

Ensuring data protection in solid state drives (SSDs) is vital in enterprise application scenario. However, as the reliability of their storage medium, namely, the NAND Flash, is decreasing at the same pace of the technology scaling, this activity is becoming nontrivial. The evaluation of different recovery strategies that employ complex error-correction codes and second-level error correction is becoming a de facto . In this paper, we model the endurance reliability of an advanced data protection methodology like the intradisk redundant array of independent disks (RAIDs) applied on mid-1X triple level cell NAND Flash-based SSD. The performed investigations include a parametric analysis of the uncorrectable bit-error rate. By developing a dedicated discrete-time Markov-chain model of an SSD, we evidenced that intradisk RAID5 and RAID6 allow achieving an inherent reliability level compliant with the qualification target for enterprise SSD. Finally, we provide a global picture of the disk economy when intradisk RAID is implemented.

Design of a Fault Tolerant Architecture for Private Cloud Computing Infrastructure

Objectives: Cloud computing has emerged as a cost-effective technology providing services to business-enterprise and IT industry. In this paper, we discuss a novel fault-tolerant scheme from an administration point of view that corresponds to implementation of Redundant Array of Independent Nodes (RAIN) model based architecture for ensuring fault-tolerance in a private cloud infrastructure setup. Methods: After exploring various combinations of Redundant Array of Independent Disks (RAID) and Logical Volume Manager (LVM) techniques, we propose a novel idea of having a fault-tolerant architecture for cloud infrastructure. Findings: As an alternative to the existing cloud fault-tolerant techniques which use expensive and specialized shared storage system, local server disk pool will be used for storing VM images. This architecture provides a robust infrastructure built over Internet Small Computer System Interface (iSCSI) targets and software RAID. Application: This RAIN based model can be deployed over the existing cloud infrastructure. This storage model can be used by cloud toolkits to provide fault tolerant backend for storage.

Graph Theory Towards Module-K Odd-Elegant Labelling Of Graphical Passwords

Graph labellings have been applied in many areas of science and engineering, such as in the development of redundant arrays of independent disks which incorporate redundancy utilizing erasure codes, algorithms, design of highly accurate optical gauging systems for use on automatic drilling machines, design of angular synchronization codes, design of optimal component layouts for certain circuit-board geometries, and determining configurations of simple resistor networks which can be used to supply any of a specified set of resistance values. Based on the idea of of “topological structure plus number theory” and magic type of various labellings for solving network transfer delay by using other type of graphical passwords. We define a new graph labelling, called Module-K odd-elegant labelling , and find some network models that admit our new labelling, and furthermore our methods can be transformed into effective algorithms.

An Effective RAID Data Layout for Object‐Based De‐duplication Backup System

We propose an alternative Redundant array of independent disks (RAID) data layout, Asymmetrical grouping data organization (AGDO), for object-based data de-duplication backup system. Object-based data deduplication is an effective solution for detecting duplicate data for compound files. We designed an asymmetrical grouping strategy, the disk in the array are partitioned into different groups, and in each group parallelism data access scheme is adopted, different types of objects are stored in different groups with maybe different group size, it can set and manage dynamic group size by using dynamic disk group adjustment algorithms. The performance of AGDO is evaluated and proved to be sufficient for the continuous storage application. The result is that disk accesses are concentrated in a part of the disks over a long time period and reduces the power consumption to 25% in a 10 disk configuration. Moreover, object-based de-duplication combined with AGDO has great potential in increasing data restoration speed for compound files. We have shown that this combination makes average restoration speed improved 11%.

DCS: Diagonal Coding Scheme for Enhancing the Endurance of SSD-Based RAID Arrays

To guarantee high reliability, solid-state drive (SSD)-based storage systems require data redundancy schemes, e.g., redundant array of independent disks (RAID) schemes. Traditional RAID-5, RAID-6, and Reed-Solomon codes can tolerate one, two, and an arbitrary number of device failures, respectively. However, some SSDs under those redundant configurations may age much faster than others because of the high skewness and locality of workloads. The uneven aging rates may make some SSDs wear out very quickly and decrease the endurance of SSD-based RAID arrays. To address this problem, we first come up with a diagonal coding scheme (DCS) by distributing the updating dependencies evenly among devices to improve the system-level wear-leveling. DCS can efficiently improve the array endurance if requests are aligned with the stripe size, i.e., when data symbols in the same stripe receive the same number of writes, while the number could be different for different stripes. To relax the above assumption, we further propose an enhanced scheme, DCS+. With a buffer design, DCS+ can improve the wear-leveling among devices under general access patterns via triggering different responses to different kinds of requests. We conduct extensive trace-driven evaluations based on real-world workloads, and results show that our design efficiently enhances the endurance of SSD-based RAID arrays.

Archiving and retrieval of experimental data using SAN based centralized storage system for SST-1

SAN (Storage Area Network, a high-speed, block level storage device) based centralized data storage system of SST-1 (Steady State superconducting Tokamak) has envisaged to address the need of availability of SST-1 operation & experimental data centrally for archival as well as retrieval [2]. Considering the initial data volume requirement, ∼10TB (Terabytes) capacity of SAN based data storage system has configured/installed with optical fiber backbone with compatibility considerations of existing Ethernet network of SST-1.The SAN based data storage system has been designed/configured with 3-tiered architecture and GFS (Global File System) cluster file system with multipath support. Tier-1 is of ∼3TB (frequent access and low data storage capacity) comprises of Fiber channel (FC) based hard disks for optimum throughput. Tier-2 is of ∼6TB (less frequent access and high data storage capacity) comprises of SATA based hard disks. Tier-3 will be planned later to store offline historical data. In the SAN configuration two tightly coupled storage servers (with cluster configuration) are working together to achieve increase performance, reliability, distribute workload and provides access to the files from any server regardless of the physical location of the file. Different RAID (Redundant Array of Independent Disks) groups are created with both, FC and SATA hard disks to increase reliability, security and performance of the storage system.The adopted SAN based data storage for SST-1 is a modular, robust, and allows future expandability. The storage modules can be added as and when required without changing the existing storage architecture. The data read/write time of the configured system is adequate enough to cater the present throughput requirements of individual subsystems. Sufficient redundancy in terms of hardware has been incorporated to assure the uninterrupted experimental data availability 24×7 within intranet. The installed storage system can be expanded up to ∼100TB of capacity with the existing controller pair presents in the SAN configuration.

RAID (Redundant Array of independent Disks)

RAID (Redundant Array of independent Disks)

Internal Parallelism of Flash Memory-Based Solid-State Drives

A unique merit of a solid-state drive (SSD) is its internal parallelism . In this article, we present a set of comprehensive studies on understanding and exploiting internal parallelism of SSDs. Through extensive experiments and thorough analysis, we show that exploiting internal parallelism of SSDs can not only substantially improve input/output (I/O) performance but also may lead to some surprising side effects and dynamics. For example, we find that with parallel I/Os, SSD performance is no longer highly sensitive to access patterns (random or sequential), but rather to other factors, such as data access interferences and physical data layout. Many of our prior understandings about SSDs also need to be reconsidered. For example, we find that with parallel I/Os, write performance could outperform reads and is largely independent of access patterns, which is opposite to our long-existing common understanding about slow random writes on SSDs. We have also observed a strong interference between concurrent reads and writes as well as the impact of physical data layout to parallel I/O performance. Based on these findings, we present a set of case studies in database management systems, a typical data-intensive application. Our case studies show that exploiting internal parallelism is not only the key to enhancing application performance, and more importantly, it also fundamentally changes the equation for optimizing applications. This calls for a careful reconsideration of various aspects in application and system designs. Furthermore, we give a set of experimental studies on new-generation SSDs and the interaction between internal and external parallelism in an SSD-based Redundant Array of Independent Disks (RAID) storage. With these critical findings, we finally make a set of recommendations to system architects and application designers for effectively exploiting internal parallelism.

H-Scale

To satisfy the explosive growth of data in large-scale data centers, where redundant arrays of independent disks (RAIDs), especially RAID-5, are widely deployed, effective storage scaling and disk expansion methods are desired. However, a way to reduce the data migration overhead and maintain the reliability of the original RAID are major concerns of storage scaling. To address these problems, we propose a new RAID scaling scheme, H-Scale, to achieve fast RAID scaling via hybrid stripe layouts. H-Scale takes advantage of the loose restriction of stripe structures to choose migrated data and to create hybrid stripe structures. The main advantages of our scheme include: (1) dramatically reducing the data migration overhead and thus speeding up the scaling process, (2) maintaining the original RAID’s reliability, (3) balancing the workload among disks after scaling, and (4) providing a general scaling approach for different RAID levels. Our theoretical analysis show that H-Scale outperforms existing scaling solutions in terms of data migration, I/O overheads, and parity update operations. Evaluation results on a prototype implementation demonstrate that H-Scale speeds up the online scaling process by up to 60% under SPC traces, and similar improvements on scaling time and user response time are also achieved by evaluations using standard benchmarks.

Generic RAID reassembly using block-level entropy

RAIDs (Redundant Array of Independent Disks) are widely used in storage systems to prevent data loss in case of hardware defects on a hard disk and to improve I/O performance. In case the RAID controller fails or in the context of a forensic investigation, the content of the RAID has to be reconstructed from the single disks or rather from disk images. Due to the variety of RAID controllers and various implementation and configuration possibilities, different parameters that are necessary for reconstruction are often unknown. This might be the case because the original configuration just has not been documented or in the forensic case, the administrator might not be cooperating and not willing to reveal the configuration. Using the original RAID system in such cases is not an option, too, because the original evidence should not be altered. We present a novel approach to automatically detect all parameters to reassemble the logical RAID volume based on block level entropy measurement and generic heuristics. We also provide a performance-optimized open source implementation of our approach that is also able to afterwards reassemble the entire logical RAID volume and to further recover single missing disks using the redundancy information as present in RAID-5.

The Architecture of High-Capacity Storage Systems

The computing industry is one of the fastest growing industries and it is caused by the rapid progression of computer hardware and software technological developments. The high-capacity storage systems become the important element of the network architectures. The network architectures of high-capacity storage systems can be developed by using various, different techniques. The present paper aims to give a review about the storage systems architecture, systematize the various types of connection, and present the significance of virtualization. Based on industry specific literature and on own research, the present paper sets up a clear systematization of these techniques by providing a brief summary of their essence, while also providing examples of their application in different network storages. The present article also provides an insight into the topic of the different connecting types of network systems, while highlighting the importance of virtualization and application of the iSCSI technology. After the explanation of the challenge of redundancies in storage systems, the author outlines the importance of using RAID (Redundant Array of Independent Disks) technology for handling the issue of redundancy

Parity Resynchronization using a Block-level Journaling for Software RAID

Software redundant arrays of independent disks (RAID) suffer from several hours of resynchronization time after a sudden power-off. Data blocks and a parity block in a stripe must be updated in a consistent manner. However, a data block may be updated without a parity update if power goes off. Such a partially modified stripe must be updated with a correct parity block after a reboot. It is difficult, however, to find which stripe is partially updated. The widely used traditional parity resynchronization approach entails a very long process that scans the entire volume to find and fix partially updated stripes. As a remedy to this problem, this paper presents a parity resynchronization scheme that exhibits a small overhead for a wide range of workloads, finishes parity resynchronization within several minutes, and is transparent to file systems, thanks to a new seamless block-level journaling. The proposed scheme is integrated into a software RAID driver in a Linux system. A performance evaluation demonstrates that the proposed scheme shortens the resynchronization process from 200min to 30s with 1% overhead, compared to 51% overhead for the prior scheme.

An efficient data layout scheme for better I/O balancing in RAID-6 storage systems

Among redundant arrays of independent disks (RAID)-6 codes, maximum distance separable (MDS) based RAID-6 codes are popular because they have the optimal storage efficiency. Although vertical MDS codes exhibit better load balancing compared to horizontal MDS codes in partial stripes, an I/O unbalancing problem still exists in some vertical codes. To address this issue, we propose a novel efficient data layout, uniform P-code (UPC), to support highly balanced I/Os among P-coded disk arrays (i.e., PC). In UPC, the nonuniformly distributed information symbols in each parity chain of P-code are moved along their columns to other rows, thus enabling the parity chain to keep original parity relationships and tolerate double disk failures. The UPC scheme not only achieves optimal storage efficiency, computational complexity, and update complexity, but also supports better I/O balancing in the context of large-scale storage systems. We also conduct a performance study on reconstruction algorithms using an analytical model. Besides extensive theoretical analysis, comparative performance experiments are conducted by replaying real-world workloads under various configurations. Experimental results illustrate that our UPC scheme significantly outperforms the PC scheme in terms of average user response time. In particular, in the case of a 12-disk array, the UPC scheme can improve the access performance of the RAID-6 storage system by 29.9% compared to the PC scheme.