Flash memory has become the most important storage media in mobile devices, and is beginning to replace hard disks in desktop systems. However, its relatively poor random write performance may cause problems in the desktop environment, which has much more complicated requirements than mobile devices. While a RAM buffer has been quite successful in hard disks to mask the low efficiency of random writes, managing such a buffer to fully exploit the characteristics of flash storage has still not been resolved. In this paper, we propose a new write buffer management scheme called Block Padding Least Recently Used, which significantly improves the random write performance of flash storage. We evaluate the scheme using trace-driven simulations and experiments with a prototype implementation. It shows about 44 % enhanced performance for the workload of MS Office 2003 installation. 1

This paper presents the design, implementation and evaluation of Direct File System (DFS) for virtualized flash storage. Instead of using traditional layers of abstraction, our layers of abstraction are designed for directly accessing flash memory devices. DFS has two main novel features. First, it lays out its files directly in a very large virtual storage address space provided by FusionIO’s virtual flash storage layer. Second, it leverages the virtual flash storage layer to perform block allocations and atomic updates. As a result, DFS performs better and it is much simpler than a traditional Unix file system with similar functionalities. Our microbenchmark results show that DFS can deliver 94,000 I/O operations per second (IOPS) for direct reads and 71,000 IOPS for direct writes with the virtualized flash storage layer on FusionIO’s ioDrive. For direct access performance, DFS is consistently better than ext3 on the same platform, sometimes by 20%. For buffered access performance, DFS is also consistently better than ext3, and sometimes by over 149%. Our application benchmarks show that DFS outperforms ext3 by 7% to 250 % while requiring less CPU power. 1

Abstract — Flash-based solid state disks (SSDs) are an alternative form of storage device that promises to deliver higher performance than the traditional mechanically rotating hard drives. While SSDs have seen utilization in embedded, consumer, and server computer systems, there has been little understanding of its performance effects with scientific I/O workloads. This paperprovides atrace driven performance evaluation of scientific I/O workloads on SSDs. We find that SSDs only provide modest performance gains over mechanical hard drives due to the writeintensive nature of many scientific workloads. Other workloads (likeread-mostly webservers) wouldlikelysee muchlarger gains. Additionally, we observe that the concurrent I/O (when multiple parallel processes simultaneously access a single storage device) may significantly affect the SSD performance. However, such effects appear to be dependent on specific SSD implementation features and they are hard to predict in a general fashion. These results suggest that abundant cautions are needed when supporting high-performance scientific I/O workloads on Flashbased SSDs. I.

With the decreasing price of flash memory, systems will increasingly use solid-state storage for virtual-memory paging rather than disks. FlashVM is a system architecture and a core virtual memory subsystem built in the Linux kernel that uses dedicated flash for paging. FlashVM focuses on three major design goals for memory management on flash: high performance, reduced flash wear out for improved reliability, and efficient garbage collection. FlashVM modifies the paging system along code paths for allocating, reading and writing back pages to optimize for the performance characteristics of flash. It also reduces the number of page writes using zero-page sharing and page sampling that prioritize the eviction of clean pages. In addition, we present the first comprehensive description of the usage of the discard command on a real flash device and show two enhancements to provide fast online garbage collection of free VM pages. Overall, the FlashVM system provides up to 94 % reduction in application execution time and is four times more responsive than swapping to disk. Furthermore, it improves reliability by writing up to 93 % fewer pages than Linux, and provides a garbage collection mechanism that is up to 10 times faster than Linux with discard support. 1

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.

Abstract—Solid State Drives (SSD’s) have shown promise to be a candidate to replace traditional hard disk drives, but due to certain physical characteristics of NAND flash, there are some challenging areas of improvement and further research. We focus on the layout and management of the small amount of RAM that serves as a cache between the SSD and the system that uses it. Of the techniques that have previously been proposed to manage this cache, we identify several sources of inefficient cache space management due to the way pages are clustered in blocks and the limited replacement policy. We develop a hybrid page/block architecture along with an advanced replacement policy, called BPAC, or Block-Page Adaptive Cache, to exploit both temporal and spatial locality. Our technique involves adaptively partitioning the SSD on-disk cache to separately hold pages with high temporal locality in a page list and clusters of pages with low temporal but high spatial locality in a block list. We run trace-driven simulations to verify our design and find that it outperforms other popular flash-aware cache schemes under different workloads. I.

Abstract—Solid-State Drives (SSDs) offer significant performance improvements over hard disk drives (HDD) on a number of workloads. The frequency of garbage collection (GC) activity is directly correlated with the pattern, frequency, and volume of write requests, and scheduling of GC is controlled by logic internal to the SSD. SSDs can exhibit significant performance degradations when garbage collection (GC) conflicts with an ongoing I/O request stream. When using SSDs in a RAID array, the lack of coordination of the local GC processes amplifies these performance degradations. No RAID controller or SSD available today has the technology to overcome this limitation. This paper presents Harmonia, a Global Garbage Collection (GGC) mechanism to improve response times and reduce performance variability for a RAID array of SSDs. Our proposal includes a high-level design of SSD-aware RAID controller and GGC-capable SSD devices, as well as algorithms to coordinate the global GC cycles. Our simulations show that this design improves response time and reduces performance variability for a wide variety of enterprise workloads. For bursty, write dominant workloads response time was improved by 69 % while performance variability was reduced by 71%. I.

As demand for mobile devices increases, prolonging battery life has been a focus of mobile device manufacturers. While manufactures support a partial self-refresh capability for MSDRAMs (Mobile SDRAM), most operating systems do not include this feature due to the complexity of the memory management. Utilizing this capability correctly could potentially reduce the amount of power consumed by MSDRAMs while a system is in a suspended mode. The goal of this project focuses on implementing the partial self-refresh capability on ARM Linux and saving the maximum amount of power while the system is in the suspended mode. 1

NANDFS is a flash file system that exposes a memory-performance tradeoff to system integrators. The file system can be configured to use a large amount of RAM, in which case it delivers excellent performance. In particular, when NANDFS is configured with the same amount of RAM that YAFFS2 uses, the performance of the two file systems is comparable (YAFFS2 is a file system that is widely used in embedded Linux and other embedded environments). But YAFFS2 and other state-of-the-art flash file systems allocate RAM dynamically and do not provide the system builder with a way to limit the amount of memory that they allocate. NAND-FS, on the other hand, allows the system builder to configure it to use a specific amount of RAM. The performance of NANDFS degrades when the amount of RAM it uses shrinks, but the degradation is graceful, not catastrophic. NANDFS is able to provide this flexibility thanks to a novel data structure that combines a coarsegrained logical-to-physical mapping with a log-structured file system.

Abstract—Storage Class Memory (SCM) has become increasingly popular in enterprise systems as well as embedded and mobile systems. However, replacing hard drives with SCMs in current storage systems often forces either major changes in file systems or suboptimal performance, because the current block-based interface does not deliver enough information to the device to allow it to optimize data management for specific device characteristics such as the out-of-place update. To alleviate this problem and fully utilize different characteristics of SCMs, we propose the use of an object-based model that provides the hardware and firmware the ability to optimize performance for the underlying implementation, and allows drop-in replacement for devices based on new types of SCM. We discuss the design of object-based SCMs and implement an object-based flash memory prototype. By analyzing different design choices for several subsystems, such as data placement policies and index structures, we show that our object-based model provides comparable performance to other flash file systems while enabling advanced features such as object-level reliability. I.