Flash Memory based Solid State Drive (SSD) has been called a “pivotal technology ” that could revolutionize data storage systems. Since SSD shares a common interface with the traditional hard disk drive (HDD), both physically and logically, an effective integration of SSD into the storage hierarchy is very important. However, details of SSD hardware implementations tend to be hidden behind such narrow interfaces. In fact, since sophisticated algorithms are usually, of necessity, adopted in SSD controller firmware, more complex performance dynamics are to be expected in SSD than in HDD systems. Most existing literature or product specifications on SSD just provide high-level descriptions and standard performance data, such as bandwidth and latency. In order to gain insight into the unique performance characteristics

We examine the write endurance of USB flash drives using a range of approaches: chip-level measurements, reverse engineering, timing analysis, whole-device endurance testing, and simulation. The focus of our investigation is not only measured endurance, but underlying factors at the level of chips and algorithms—both typical and ideal—which determine the endurance of a device. Our chip-level measurements show endurance far in excess of nominal values quoted by manufacturers, by a factor of as much as 100. We reverse engineer specifics of the Flash Translation Layers (FTLs) used by several devices, and find a close correlation between measured whole-device endurance and predictions from reverse-engineered FTL parameters and measured chip endurance values. We present methods based on analysis of operation latency which provide a non-intrusive mechanism for determining FTL parameters. Finally we present Monte Carlo simulation results giving numerical bounds on endurance achievable by any on-line algorithm in the face of arbitrary or malicious access patterns. 1

We present FlashStore, a high throughput persistent keyvalue store, that uses flash memory as a non-volatile cache between RAM and hard disk. FlashStore is designed to store the working set of key-value pairs on flash and use one flash read per key lookup. As the working set changes over time, space is made for the current working set by destaging recently unused key-value pairs to hard disk and recycling pages in the flash store. FlashStore organizes key-value pairs in a log-structure on flash to exploit faster sequential writeperformance. Itusesanin-memoryhashtabletoindex them, with hash collisions resolved by a variant of cuckoo hashing. The in-memory hash table stores compact key signatures instead of full keys so as to strike tradeoffs between RAM usage and false flash read operations. FlashStore can be used as a high throughput persistent key-value storage layer for a broad range of server class applications. We compare FlashStore with BerkeleyDB, an embedded key-value store application, running on hard disk and flash separately, so as to bring out the performance gain of FlashStore in not only using flash as a cache above hard disk but also in its use of flash aware algorithms. We use real-world data traces from two data center applications, namely, Xbox LIVE Primetime online multi-player game and inline storage deduplication, to drive and evaluate the design of FlashStore on traditional and low power server platforms. FlashStore outperforms BerkeleyDB by up to 60x on throughput (ops/sec), up to 50x on energy efficiency (ops/Joule), and up to 85x on cost efficiency (ops/sec/dollar) on the evaluated datasets. 1.

Abstract—Solid state disks (SSDs) consisting of NAND flash memory are being widely used in laptops, desktops, and even enterprise servers. SSDs have many advantages over hard disk drives (HDDs) in terms of reliability, performance, durability, and power efficiency. Typically, the internal hardware and software organization varies significantly from SSD to SSD and thus each SSD exhibits different parameters which influence the overall performance. In this paper, we propose a methodology which can extract several essential parameters affecting the performance of SSDs. The target parameters of SSDs considered in this paper are (1) the size of read/write unit, (2) the size of erase unit, (3) the type of NAND flash memory used, (4) the size of read buffer, and (5) the size of write buffer. Obtaining these parameters will allow us to understand the internal architecture of the target SSD better and to get the most performance out of SSD by performing SSDspecific optimizations. 1 I.

With the fast technical improvement, flash memory based Solid State Drives (SSDs) are becoming an important part of the computer storage hierarchy to significantly improve performance and energy efficiency. However, due to its relatively high price and low capacity, a major system research issue to address is on how to make SSDs play their most effective roles in a high-performance storage system in cost- and performance-effective ways. In this paper, we will answer several related questions with insights based on the design and implementation of a high performance hybrid storage system, called Hystor. We make the best use of SSDs in storage systems by achieving a set of optimization objectives from both system deployment and algorithm design perspectives. Hystor manages both SSDs and hard disk drives (HDDs) as one single block device with minimal changes to existing OS kernels. By monitoring I/O access patterns at runtime, Hystor can effectively identify blocks that (1) can result in long latencies or (2) are semantically critical (e.g. file system metadata), and stores them in SSDs for future accesses to achieve a significant performance improvement. In order to further leverage the exceptionally high performance of writes in the state-of-the-art SSDs, Hystor also serves as a write-back buffer to speed up write requests. Our measurements on Hystor implemented in the Linux kernel 2.6.25.8 show that it can take advantage of the performance merits of SSDs with only a few lines of changes to the stock Linux kernel. Our system study shows that in a highly effective hybrid storage system, SSDs should play a major role as an independent storage where the best suitable data are adaptively and timely migrated in and retained, and it can also be effective to serve as a write-back buffer.

Flash memory is the largest change to storage in recent history. Most research to date has focused on integrating flash as persistent storage in file systems, with little emphasis on virtual memory paging. However, the VM architecture in most of the commodity operating systems is heavily customized for using disks through software layering, request clustering, and prefetching. We revisit the VM hierarchy in light of flash memory and identify mechanisms that inhibit utilizing its full potential. We find that software latencies for a page fault could be as high as the time taken to read a page from flash, and that swap systems are overly tuned towards the characteristics of disks. Based on this study, we propose a new system design, FlashVM, that pages directly to flash memory, avoids unnecessary disk-based optimizations, and orders page writes to flash memory without any firmware support. With flash prices dropping exponentially and speeds improving, we argue that FlashVM can support memory intensive applications more economically than conventional DRAM-based systems.

NAND Flash memory-based Solid-State Disks (SSDs) are becoming popular as the storage media in domains ranging from mobile laptops to enterprise-scale storage systems due to a number of benefits (e.g., lighter weights, faster access times, lower power consumption, higher resistance to vibrations) they offer over the conventionally popular Hard Disk Drives (HDDs). While a number of well-regarded simulation environments exist for HDDs, the same is not yet true for SSDs. This is due to SSDs having been in the storage market for relatively less time as well as the lack of information (hardware configuration and software methods) about state-of-the-art SSDs that is publicly available. We describe the design and implementation of FlashSim, a simulator aimed at filling this void in performance evaluation of emerging storage systems that employ SSDs. FlashSim is an event-driven simulator that follows the objected-oriented programming paradigm for modularity. We have validated the performance of FlashSim against a number of commercial SSDs for behavioral similarity. We have also used FlashSim to compare the performance of SSD devices employing different Flash Translation Layer (FTL) schemes, and analyzed the energy consumption of different FTL schemes in the SSD. FlashSim has been written to be inter-operable with the well-regarded DiskSim simulator, thus enabling the simulation of a variety of “hybrid ” storage systems employing combinations of SSDs and HDDs. Given the current interest in such hybrid systems as opposed to systems with SSDs replacing HDDs (due to higher price), we believe this to be an especially useful feature of FlashSim. We have made FlashSim freely available for download with the hope that it would be of use to researchers exploring the design of SSD-based systems. 1

We present nameless writes, a new interface that obviates the need for indirection in modern solid-state storage devices (SSDs). Nameless writes allow the device to pick the location of a write and only then inform the client above of the decision. Doing so keeps control of block allocation decisions in the device, thus enabling it to perform important tasks such as wear-leveling, while removing the need for large and costly indirection tables. We discuss the proposed interface as well as the requisite device and file-system support. 1

The availability of high-speed solid-state storage has introduced a new tier into the storage hierarchy. Low-latency and high-IOPS solid-state drives (SSDs) cache data in front of high-capacity disks. However, most existing SSDs are designed to be a drop-in disk replacement, and hence are mismatched for use as a cache. This paper describes FlashTier, a system architecture built upon solid-state cache (SSC), a flash device with an interface designed for caching. Management software at the operating system block layer directs caching. The FlashTier design addresses three limitations of using traditional SSDs for caching. First, FlashTier provides a unified logical address space to reduce the cost of cache block management within both the OS and the SSD. Second, FlashTier provides cache consistency guarantees allowing the cached data to be used following a crash. Finally, FlashTier leverages cache behavior to silently evict data blocks during garbage collection to improve performance of the SSC. We have implemented an SSC simulator and a cache manager in Linux. In trace-based experiments, we show that FlashTier reduces address translation space by 60 % and silent eviction improves performance by up to 167%. Furthermore, FlashTier can recover from the crash of a 100 GB cache in only 2.4 seconds.

We present Nameless Writes, a new device interface that removes the need for indirection in modern solid-state storage devices (SSDs). Nameless writes allow the device to choose the location of a write; only then is the client informed of the name (i.e., address) where the block now resides. Doing so allows the device to control blockallocation decisions, thus enabling it to execute critical tasks such as garbage collection and wear leveling, while removing the need for large and costly indirection tables. We demonstrate the effectiveness of nameless writes by porting the Linux ext3 file system to use an emulated nameless-writing device and show that doing so both reduces space and time overheads, thus making for simpler, less costly, and higher-performance SSD-based storage. 1