In Hwan Doh

Exploiting non-volatile RAM to enhance flash file system performance

Non-volatile RAM (NVRAM) such as PRAM (Phase-change RAM), FeRAM (Ferroelectric RAM), and MRAM (Magnetoresistive RAM) has characteristics of both non-volatile storage and random access memory (RAM). These forms of NVRAM are currently being developed by major semiconductor companies and are expected to be an everyday component in the near future. The advent of NVRAM may possibly bring about drastic changes to the system software landscape. In this work, we develop a new Flash memory based file system that exploits NVRAM in order to improve system performance. Specifically, we discuss the initial design and implementation of a file system that stores all metadata in NVRAM, while storing all file data in Flash memory. In so doing, we make two contributions in this work. First, we present a model that analyzes the amount of NVRAM that is needed for specific Flash memory storage capacity. Experimentally, we verify that this model represents the exact NVRAM usage in the realistic environment. Second, we present quantitative experimental results that show how much performance gains are possible by exploiting NVRAM. Compared to YAFFS, a popular Flash memory based file system, we show that this file system requires only minimal time for mounting and that the execution time improves by a maximum of 600% and an average of 437% for the realistic workloads that we considered.

Impact of NVRAM write cache for file system metadata on I/O performance in embedded systems

File systems make use of part of DRAM as the buffer cache to enhance its performance in traditional systems. In this paper, we consider the use of Non-Volatile RAM (NVRAM) as a write cache for metadata of the file system in embedded systems. NVRAM is a state-of-the-art memory that provides characteristics of both non-volatility and random byte addressability. By making NVRAM a write cache for dirty metadata, we retain the same integrity of a file system that always synchronously writes its metadata to storage, while at the same time improving file system performance to the level of a file system that always writes asynchronously. To show quantitative results, we develop an embedded board with NVRAM and modify the VFAT file system provided in Linux 2.6.21 to accommodate the NVRAM write cache. The experimental results show that substantial reductions in execution time are possible from an application viewpoint. Another consequence of the write cache is its benefits at the FTL layer, leading to improved wear leveling of Flash memory and increased energy savings, which are important measures in embedded systems.

Towards greener data centers with storage class memory

Studies have shown that much of todays data centers are over-provisioned and underutilized. Over-provisioning cannot be avoided as these centers must anticipate peak load with bursty behavior. Under-utilization, to date, has also been unavoidable as systems always had to be ready for that sudden burst of requests that loom just around the corner. Previous research has pointed to turning off systems as one solution, albeit, an infeasible one due to its irresponsiveness. In this paper, we present the feasibility of using new Storage Class Memory (SCM, which encompasses specific developments such as PCM, MRAM, or FeRAM) technology to turn systems on and off with minimum overhead. This feature is used to control systems on the whole (in comparison to the previous fine-grained component-wise control) in finer time scale for high responsiveness with minimized power lost to idleness. Our empirical study is done by executing real trace-like workloads on a prototype data center of embedded systems deploying FeRAM. We quantify the energy savings and performance trade-off by turning idle systems off. We show that our energy savings approach consumes energy in proportion to user requests with configurable quality of service. Finally, based on observations made on this data center, we discuss the requirements for real deployment.

An Empirical Study of Deploying Storage Class Memory into the I/O Path of Portable Systems

We explore the possibility of deploying storage class memory (SCM) into the I/O path as a file system metadata store and examine what effects it has on the performance of portable computing systems. In this regard, we develop a new flash memory-based file system that stores all metadata in SCM, while storing all file data in flash memory. In so doing, we make two contributions in this work. First, we present a model that analyzes the amount of SCM that is needed for specific flash memory storage capacity. Second, we present quantitative experimental results that show how much performance gains are possible by exploiting SCM in terms of I/O performance, energy efficiency and lifetime of the underlying flash memory. Compared to YAFFS, a popular flash memory-based file system, we show that system performance is improved by a maximum of around 320, 260, and 180% in terms of I/O performance, energy efficiency and lifetime of the Flash memory, respectively, for the realistic workloads that we considered.

RPP: reference pattern based prefetching controller

We propose a prefetching technique that recognizes and exploits reference patterns. Reference Pattern based Prefetching (RPP) detects sequential and non-sequential reference patterns and takes different prefetch action according to this information. Beyond sequential detection, RPP recognizes repetitive occurences of consecutive sequences, and then exploits these repetitions. In this paper, we show that RPP results in favorable performance gains.