|Title:||Hardware/software codesign for performance and lifetime enhancement in NAND-flash-based embedded storage systems|
|Advisors:||Shao, Zili (COMP)|
|Subject:||Embedded computer systems -- Design and construction|
Memory management (Computer science)
Flash memories (Computers)
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Computing|
|Pages:||xvi, 121 pages : color illustrations|
|Abstract:||Embedded systems (e.g. smartphones) have become an integral part in peoples' daily life. In the last several years, most of efforts in improving embedded systems have been focusing on enhancing the network performance and CPU speed. On the other hand, the performance of NAND-flash-based storage in embedded systems is stagnant, and has become one of the major performance bottlenecks in embedded systems. Even worse, the performance and lifetimes ofNAND flash memory are substantially degraded with the advent of multi-level cell and triple-level cell flash memory for increasing the capacity. In this thesis, we have addressed these issues from several aspects including the integration of the emerging hardware and the cross-layer software management for the optimization of the performance and lifetime. First, we focus on employing the self-healing NAND flash memory to improve the lifetime and performance. Researchers have recently discovered that heating can cause worn-out NAND flash cells to become reusable and greatly extend the lifetime of flash memory cells. However, the heating process consumes a substantial amount of power, and some fundamental changes are required for existing NAND flash management techniques. In particular, all existing wear-leveling techniques are based on the principle of evenly distributing writes and erases. For self-healing NAND flash, this may cause NAND flash cells to be worn out in a short period of time. Moreover, frequently healing these cells may drain the energy quickly in battery-driven mobile devices, which is defined as the concentrated heating problem. We propose a novel wear-leveling scheme called DHeating (Dispersed Heating) to address the problem. In DHeating, rather than evenly distributing writes and erases over a time period, write and erase operations are scheduled on a small number of flash memory cells at a time, so that these cells can be worn out and healed much earlier than other cells. In this way, we can avoid quick energy depletion caused by concentrated heating. In addition, the heating process takes several seconds and has become the new performance bottleneck. In order to address this issue, we propose a lazy heating repair scheme. The lazy heating repair scheme can ease the long time heating effect by delaying the heating operation and using the system idle time to repair. Furthermore, the flash memory's reliability becomes worse with the flash memory cells reaching the expected worn-out time. We propose an early heating strategy to solve the reliability problem. With the extended lifetime provided by self-healing, we can trade some lifetimes for reliability. The idea is to start the healing process earlier than the expected worn-out time. We evaluate our scheme based on an embedded platform. The experimental results show that the proposed scheme can effectively prolong the consecutive heating time interval, alleviate the long time heating effect, and enhance the reliability for self-healing flash memory.|
Second, we jointly optimize the NAND flash memory's lifetime and performance with the integration of NVMs. Novel NVMs (non-volatile memories), such as PCM (Phase Change Memory) and STT-RAM (Spin-TransferTorque Random Access Memory), can provide fast read/write operations. In this thesis, we propose a unified NVM/flash architecture to improve the I/O performance. A transparent scheme, vFlash (Virtualized Flash), is also proposed to manage the unified architecture. Within vFlash, inter-app and intra-app techniques are proposed to optimize the application performance by exploiting the historical locality and I/O access patterns of applications. Since vFlash is on the bottom of the I/O stack, the application features willbe lost. Therefore, we also propose a cross-layer technique to transfer the application information from the application layer to the vFlash layer. The proposed scheme is evaluated based on an Android platform, and the experimental results show that the proposed scheme can effectively improve the I/O performance of mobile devices. Third, we study the problem of performance and lifetime enhancement in the mobile virtualization environment. Mobile virtualization introduces extra layers in software stacks, which leads to performance degradation. Especially, each I/O operation has to pass through several software layers to reach the NAND-flash-based storage systems. This thesis targets at optimizing I/O for mobile virtualization, since I/O becomes one of major performance bottlenecks that seriously affects the performance of mobile devices. Among all the I/O operations, a large percentage is updating metadata. Frequent updating metadata not only degrades overall I/O performance but also severely reduces flash memory lifetime. In this thesis, we propose a novel I/O optimization technique to identify the metadata of a guest file system which is stored in a VM (Virtual Machine) image file and frequently updated. Then, these metadata are stored in a small additional NVM (Non-Volatile Memory) which is faster and more endurable to greatly improve flash memory's performance and lifetime. To the best of our knowledge, this is the first work to identify the file system metadata from regular data in a guest OS VM image file under mobile virtualization. The proposed scheme is evaluated on a real hardware embedded platform. The experimental results show that the proposed techniques can improve write performance to 45.21% in mobile devices with virtualization.
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