First Evidence of Temporary Read Errors in TLC 3D-NAND Flash Memories Exiting From an Idle State

This paper presents a new reliability threat that affects 3D-NAND Flash memories when a read operation is performed exiting from an idle state. In particular, a temporary large increase of the fail bits count is reported for the layers read as first after a sequence of program/verify and a idle retention phase. The phenomenon, hereafter called Temporary Read Errors (TRE), is not due to a permanent change of cell threshold voltage between the program verify and the following read operations, but to its transient instability occurring during the idle phase and the first read operations performed on a block. The experimental analysis has been performed on off-the-shelf gigabit-array products to characterize the dependence on the memory operating conditions. The TRE is found to be strongly dependent on the page read, on the read temperature and on the time delay between the first and the second read after the idle state. To emphasize its negative impact at system-level, we have evaluated the induced performance drop on Solid State Drives architectures

Memory-Driven Design Methodologies For Solid State Drives (SSDs)

The unparalleled cost and form factor advantages of NAND flash memory has driven 35mm photographic film, floppy disks and one inch hard drives to extinction. NAND Flash memory is now expanding its reach in the form of Solid State Drives (SSDs). The basic architecture of SSDs is discussed in Chapter 1. SSDs’ performance and reliability are strictly linked to that of the NAND Flash memories. For this reason, the SSD design flow must follow a bottom up approach that, starting from an accurate knowledge of the reliability of NAND Flash memories, selects the most appropriate error correction strategy to extend the SSD’s lifetime. Chapter 2 will discuss this bottom up approach To fuel the transition from HHD to SSD, NAND must remain very aggressive in terms of ost per bit. When approaching 10 nm technologies, planar NAND is running out of steam. 3D integration turned out to be the most promising alternative. Chapter 3 is about 3D NAND Flash memories. The advent of 3D NAND has introduced significant issues in terms of characterization and system level optimization. In Chapter 4 we’ll show how machine learning algorithms can help designers to optimize the memory reliability. NAND Flash memories are complex systems. Many efforts in the reliability community are devoted to investigating the reliability loss of this storage medium from a cell’s physics point of view. In Chapter 5 we present a different reliability threat related to the high voltage circuitry of the memory: the dependence from the power supply The read disturb is another important problem related to TLC NAND Flash memories since their usage model is predominantly based on read intensive applications. The state of the art qualification methods of Flash memories are performed by uniformly stressing the memory blocks with the same amount of reads. However, by analyzing several workloads, it appears that the read operations can also be concentrated in a specific address range. In Chapter 6, we’ll show the different behavior of a mid 1X TLC NAND Flash under uniform and concentrated read disturb Flash technology in not the only possible medium for SSDs. RRAM is perceived as a reliable alternative to NAND Flash in SSDs for low latency applications. These emerging memories are non volatile as NAND Flash, but with a lower read/write latency and a higher reliability. However, the relatively small storage capacity of RRAM memories integrated so far has limited their usage to specific applications such as saving critical data during power loss events. In Chapter 7 a design space exploration of a 512 GB All RRAM SSD architecture is performed by using a custom developed simulator PCIe DRAM/Flash based NVRAM (Non Volatile RAM) cards are gaining traction in the market because they can be used either as a very fast and secure synchronous write buffer, or to store both critical system data and user data in case of Power Failure. In a nutshell, the host sees the NVRAM card as a bunch of DRAM devices connected over a PCIe bus. If the power suddenly disappears, the on board controller copies the DRAM content to a bank of Flash memories; during this copy operation, a super capacitor supplies the necessary energy. MRAM memories are now mature enough to offer a technically viable alternative to the combination of DRAM and Flash. In Chapter 8 we present a analysis of IOPS and latency (QoS) for both DRAM/Flash based and All MRAM NVRAM cards. Ensuring data protection in Solid State Drives is vital in enterprise application . However, as the reliability of their storage medium is decreasing at the same pace of the technology scaling, this activity is becoming non trivial. In Chapter 9 we model the endurance reliability of an advanced data protection methodology like the intra disk Redundant Array of Independent Disks (RAID) applied to mid 1X Triple Level Cell NAND Flash based SSD. The performed investigations include a parametric analysis of the Uncorrectable Bit Error Rat

Solid-state-drives (SSDs) modeling: simulation tools & strategies

This book introduces simulation tools and strategies for complex systems of solid-state-drives (SSDs) which consist of a flash multi-core microcontroller plus NAND flash memories. It provides a broad overview of the most popular simulation tools, with special focus on open source solutions. VSSIM, NANDFlashSim and DiskSim are benchmarked against performances of real SSDs under different traffic workloads. PROs and CONs of each simulator are analyzed, and it is clearly indicated which kind of answers each of them can give and at a what price. It is explained, that speed and precision do not go hand in hand, and it is important to understand when to simulate what, and with which tool. Being able to simulate SSD’s performances is mandatory to meet time-to-market, together with product cost and quality. Over the last few years the authors developed an advanced simulator named “SSDExplorer” which has been used to evaluate multiple phenomena with great accuracy, from QoS (Quality Of Service) to Read Retry, from LDPC Soft Information to power, from Flash aging to FTL. SSD simulators are also addressed in a broader context in this book, i.e. the analysis of what happens when SSDs are connected to the OS (Operating System) and to the end-user application (for example, a database search). The authors walk the reader through the full simulation flow of a real system-level by combining SSD Explorer with the QEMU virtual platform. The reader will be impressed by the level of know-how and the combination of models that such simulations are asking for

Editorial for the Special Issue on Flash Memory Devices

Flash memory devices represented a breakthrough in the storage industry since their inception in the mid-1980s, and innovation is still ongoing after more than 35 years [...

Solid-State Drives: Memory Driven Design Methodologies for Optimal Performance

Solid-state drives (SSDs) faced an astonishing development in the last few years, becoming the cornerstone to new paradigms and markets of the information technology, such as cloud computing and big data centers. So far, the SSD design approach has focused on the optimization of the Flash translation layer, the firmware devoted to fulfill the compatibility with traditional hard-disk drives. For hyperscaled SSDs this strategy is no longer valid since their performance and reliability are strictly linked to that of the NAND Flash memories that constitute the storage medium, in particular when the multilevel cell paradigm is considered. For this reason, the design flow must follow a bottom-up approach that, starting from an accurate knowledge of the time and use dependent reliability of the NAND Flash memories, selects the most appropriate error correction strategy to extend the SSD lifetime while reducing its performance degradation. Then, the design flow moves to that of the SSD controller and of the interface toward the host where the application is running. This paper will thoroughly discuss this bottom-up approach, and finally, it will show how it is possible to leverage new approaches, such as the software-defined storage system that, by exploiting a hardware/software codesign of the SSD controller architecture and of the host application, will be able to revolutionize the traditional computer/memory interaction

Inside Solid State Drives (SSDs)

Solid State Drives (SSDs) are gaining momentum in enterprise and client applications, replacing Hard Disk Drives (HDDs) by offering higher performance and lower power. In the enterprise, developers of data center server and storage systems have seen CPU performance growing exponentially for the past two decades, while HDD performance has improved linearly for the same period. Additionally, multi-core CPU designs and virtualization have increased randomness of storage I/Os. These trends have shifted performance bottlenecks to enterprise storage systems. Business critical applications such as online transaction processing, financial data processing and database mining are increasingly limited by storage performance. In client applications, small mobile platforms are leaving little room for batteries while demanding long life out of them. Therefore, reducing both idle and active power consumption has become critical. Additionally, client storage systems are in need of significant performance improvement as well as supporting small robust form factors. Ultimately, client systems are optimizing for best performance/power ratio as well as performance/cost ratio. SSDs promise to address both enterprise and client storage requirements by drastically improving performance while at the same time reducing power. Inside Solid State Drives walks the reader through all the main topics related to SSDs: from NAND Flash to memory controller (hardware and software), from I/O interfaces (PCIe/SAS/SATA) to reliability, from errror correction codes (BCH and LDPC) to encryption, from Flash signal processing to hybrid storage. We hope you enjoy this tour inside Solid State Drives