Jae-Geuk Kim

A buffer replacement algorithm exploiting multi-chip parallelism in solid state disks

Solid State Disks (SSDs) are superior to magnetic disks from a performance point of view due to the favorable features of NAND flash memory. Furthermore, thanks to improvement on flash memory density and adopting a multi-chip architecture, SSDs replace magnetic disks rapidly. Most previous studies have been conducted for enhancing the performance of SSDs, but these studies have been worked on the assumption that the operation unit of a host interface is the same as the operation unit of NAND flash memory, where it is needless to give consideration to partially-filled pages. In this paper, we analyze the overhead caused by the partially-filled pages, and propose a buffer replacement algorithm exploiting multi-chip parallelism to enhance the write performance. Our simulation results show that the proposed algorithm improves the write performance by up to 30% over existing approaches.

Sector log: fine-grained storage management for solid state drives

Although NAND flash-based solid-state drives (SSDs) excel magnetic disks in several aspects, the costs of write operations have been limiting their performance. The overheads of write operations are exacerbated by the fixed write unit (page) of flash memory, which is much larger than the sector size in magnetic disks. A write request from a file system, with a data size smaller than a page, becomes a full page write in SSDs. With the page size hidden internally in SSDs, file systems and applications may not be optimized to a fixed page size. Furthermore, to increase the density and bandwidth of flash memory, page sizes in SSDs have been increasing. In this paper, we propose a sector-level data management mechanism for SSDs, called sector log. Sector log manages a small part of NAND flash memory in SSDs with sector-level mapping, and stores sub-page writes more efficiently than conventional SSDs. While current small DRAM buffers cannot absorb the working set of sub-page writes for certain applications, sector log uses ample persistent storage in flash memory. With the sector mapping mechanism, sector log provides a sector-accessible block device abstraction upon page-managed flash memory.

ScaleFFS: A scalable log-structured flash file system for mobile multimedia systems

NAND flash memory has become one of the most popular storage media for mobile multimedia systems. A key issue in designing storage systems for mobile multimedia systems is handling large-capacity storage media and numerous large files with limited resources such as memory. However, existing flash file systems, including JFFS2 and YAFFS in particular, exhibit many limitations in addressing the storage capacity of mobile multimedia systems. In this article, we design and implement a scalable flash file system, called ScaleFFS, for mobile multimedia systems. ScaleFFS is designed to require only a small fixed amount of memory space and to provide fast mount time, even if the file system size grows to more than tens of gigabytes. The measurement results show that ScaleFFS can be instantly mounted regardless of the file system size, while achieving the same write bandwidth and up to 22% higher read bandwidth compared to JFFS2.

FlashLight: A Lightweight Flash File System for Embedded Systems

A very promising approach for using NAND flash memory as a storage medium is a flash file system. In order to design a higher-performance flash file system, two issues should be considered carefully. One issue is the design of an efficient index structure that contains the locations of both files and data in the flash memory. For large-capacity storage, the index structure must be stored in the flash memory to realize low memory consumption; however, this may degrade the system performance. The other issue is the design of a novel garbage collection (GC) scheme that reclaims obsolete pages. This scheme can induce considerable additional read and write operations while identifying and migrating valid pages. In this article, we present a novel flash file system that has the following features: (i) a lightweight index structure that introduces the hybrid indexing scheme and intra-inode index logging, and (ii) an efficient GC scheme that adopts a dirty list with an on-demand GC approach as well as fine-grained data separation and erase-unit data allocation. We implemented FlashLight in a Linux OS with kernel version 2.6.21 on an embedded device. The experimental results obtained using several benchmark programs confirm that FlashLight improves the performance by up to 27.4% over UBIFS by alleviating index management and GC overheads by up to 33.8%.

Co-optimization of buffer layer and FTL in high-performance flash-based storage systems

NAND flash-based storage devices have rapidly improved their position in the secondary storage market ranging from mobile embedded systems to personal computer and enterprise storage systems. Recently, the most important issue of NAND flash-based storage systems is the performance of random writes as well as sequential writes, which strongly depends on their two main software layers: a Buffer Management Layer (BML) and a Flash Translation Layer (FTL). The primary goal of our study is to highly improve the overall performance of NAND flash-based storage systems by exploiting the cooperation between those two layers. In this paper, we propose an FTL-aware BML policy called Selective Block Padding and a BML-based FTL algorithm called Optimized Switch Merge, which overcome the limitations of existing approaches on performance enhancement. When using both the proposed techniques, evaluation results show that the throughput is significantly increased over that of previous studies.

Efficient Metadata Management for Flash File Systems

NAND flash memory becomes one of the most popular storage for portable embedded systems. Although many flash-aware file systems, such as JFFS2 and YAFFS2, were proposed, the large memory consumption and the long mount delay have been serious obstacles for large-capacity NAND flash memory. In this paper, we present a new flash-aware file system called DFFS (direct flash file system) which fetches only the needed metadata on demand from flash memory. In addition, DFFS employs two novel metadata management schemes, inode embedding scheme and hybrid inode indexing scheme, to improve the performance of metadata operations. Comprehensive evaluation results using microbench- mark, postmark, and Linux kernel compilation trace, show that DFFS has comparable performance to JFFS2 and YAFFS2, while achieving a small memory footprint and instant mount time.

RMA: A Read Miss-Based Spin-Down Algorithm using an NV Cache

It is an important issue to reduce the power consumption of a hard disk that takes a large amount of computer systempsilas power. As a new trend, an NV cache is used to make a disk spin down longer by servicing read/write requests instead of the disk. During the spin-down periods, write requests can be simply handled by write buffering, but read requests are still the main cause of initiating spin-ups because of a low hit ratio in the NV cache. Even when there is no user activity, read requests can be frequently generated by running applications and system services, hindering the spin-down. In this paper, we propose new NV cache policies: active write caching to reduce or to delay spin-ups caused by read misses during spin-down periods and a read miss-based spin-down algorithm to extend the spin-down periods, exploiting the NV cache effectively. Our policies reduce the power consumption of a hard disk by up to 50.1% with a 512 MB NV cache, compared with preceding approaches.