Seung-Ho Lim

An efficient NAND flash file system for flash memory storage

In this paper, we present an efficient flash file system for flash memory storage. Flash memory, especially NAND flash memory, has become a major method for data storage. Currently, a block level translation interface is required between an existing file system and flash memory chips due to its physical characteristics. However, the approach of existing file systems on top of the emulating block interface has many restrictions and is, thus, inefficient because existing file systems are designed for disk-based storage systems. The flash file system proposed in this paper is designed for NAND flash memory storage while considering the existing file system characteristics. Our target performance metrics are the system booting time and garbage collection overheads, which are important issues in flash memory. In our experiments, the proposed flash file system outperformed other flash file systems both in booting time and garbage collection overheads.

PFFS: a scalable flash memory file system for the hybrid architecture of phase-change RAM and NAND flash

In this paper, we present the scalable and efficient flash file system using the combination of NAND and Phase-change RAM (PRAM). Until now, several flash file systems have been developed considering the physical characteristics of NAND flash. However, previous flash file systems still have a high performance overhead and a scalability problem of the mounting time and the memory usage because, in most case, the metadata is written with several words at a single update even though the writes in NAND flash must be performed in terms of page, which is typically 2 KiB. The proposed flash file system called PFFS uses PRAM to mitigate the limitation of NAND flash. The PRAM is a next generation non-volatile memory and good for dealing with word level read/write of a small size of data. PFFS hence separates the metadata from the regular data in a file system and saves them into PRAM. Consequently, the PFFS manages all the files and directories in the PRAM and outperforms other flash file systems. The experimental results show that the performance of PFFS is 25% better than YAFFS2 for small-file writes while matching YAFFS2 performance for large writes and the mouting time and the memory usage of PFFS are O(1).

JFTL: A flash translation layer based on a journal remapping for flash memory

In flash memory-based storage, a Flash Translation Layer (FTL) manages the mapping between the logical addresses of a file system and the physical addresses of the flash memory. When a journaling file system is set up on the FTL, the consistency of the file system is guaranteed by duplications of the same file system changes in both the journal region of the file system and the home locations of the changes. However, these duplications inevitably degrade the performance of the file system. In this article we present an efficient FTL, called JFTL, based on a journal remapping technique. The FTL uses an address mapping method to write all the data to a new region in a process known as an out-of-place update. Because of this process, the existing data in flash memory is not overwritten by such an update. By using this characteristic of the FTL, the JFTL remaps addresses of the logged file system changes to addresses of the home locations of the changes, instead of writing the changes once more to flash memory. Thus, the JFTL efficiently eliminates redundant data in the flash memory as well as preserving the consistency of the journaling file system. Our experiments confirm that, when associated with a writeback or ordered mode of a conventional EXT3 file system, the JFTL enhances the performance of EXT3 by up to 20%. Furthermore, when the JFTL operates with a journaled mode of EXT3, there is almost a twofold performance gain in many cases. Moreover, the recovery performance of the JFTL is much better than that of the FTL.

A Ubiquitous Fashionable Computer with an i-Throw Device on a Location-Based Service Environment

The ubiquitous fashionable computer (UFC), introduced in this paper, is a wearable computer that allows people to exploit ubiquitous computing environment in a user-friendly manner. We present the design approach and philosophy of the UFC that is wearable, aesthetic, and intuitive. The UFC supports the interoperability of various communication interfaces among WLAN, Bluetooth and ZigBee devices. We developed a wireless gesture recognition device, called i -Throw, which is small enough to be worn on ones finger like a ring. The UFC, with the help of i -Throw, can control ubiquitous environment using an intuitive hand motion. To explain the practical use of the UFC platform and the user-friendly interaction with ubiquitous environment, we implemented a ubiquitous testbed where multiple UFC users interact with various ubiquitous devices or other UFC users. In addition, we implemented a practical application which makes it possible to exchange the various objects and control ubiquitous devices very easily.

Hashing Directory Scheme for NAND Flash File System

Flash memory, especially NAND flash memory, has become a major method for data storage in mobile systems because of its small size, shock resistance, and low power consumption. When the flash memory is used, conventional file systems cannot be used upon a bare flash memory due to physical characteristics. Instead, few flash file systems are developed which use the direct interfaces with flash memory. However, their data recording method have problems as the capacity of flash increases, because these use log based data recording method, which means that they encapsulate the metadata information into the data node for all files when data are written. Their method is good for the small capacity of flash memory, but not adequate for large sizes because it consumes more mount time, memory footprint and operation times as capacity increases. In this paper, we present an efficient metadata management scheme of flash file system for giga scale flash memory. Specifically, we design the hashing directory structure for directory management and two level index structure for file index management. These management scheme can reduce the file system mount time and memory footprint during runtime.

Interactive media server with media synchronized RAID storage system

We propose an efficient placement algorithm and per-disk prefetching method to effectively support interactive operations in the media server. Our placement policy is incorporated with an encoder having a special bitcount control scheme that repeatedly tunes quantization parameters to adjust the bitcounts of video frames. This encoder can generate coded frames sub-stream video blocks whose sizes are synchronized with the RAID stripe size, so that when various fast-forward levels are accessed we can reduce the seek and rotational latency and enhance the disk throughput of each disk in the RAID system. In the experimental results, the proposed placement policy and bitrate control scheme can significantly improve the average service time, which can enlarge the capacity of the interactive media server.

Journal remap-based FTL for journaling file system with flash memory

Constructing flash memory based storage, FTL (Flash Translation Layer) manages mapping between logical address and physical address. Since FTL writes every data to newregion by itsmappingmethod, the previous data is not overwritten by new write operation.When a journaling file system is set up upon FTL, it duplicates data between the journal region and its home location for the file system consistency. However, the duplication degrades the performance. In this paper, we present an efficient journal remap-based FTL. The proposed FTL, called JFTL, eliminates the redundant data duplication by remapping the journal region data path to home location of file system. Thus, our JFTLcan prevent from degrading write performance of file system while preserving file system consistency.

Write back routine for JFFS2 efficient i/o

When flash memory is used as a storage in embedded systems, block level translation layer is required between conventional filesystem and flash memory chips due to its physical characteristics. A far more efficient use of it is the design of a filesystem itself without no extra layer of translation. However, since flash filesystem does not use block device layer, it cannot utilize deferred I/O although deferred I/O enhances write latency by delaying the flushing jobs. Linux operating system generally uses the write back routine for deferred I/O using kernel thread, which writes back dirty pages and buffers through the block device layer. In this paper, we design and implement efficient I/O for JFFS2 flash filesystem based on flash memory. For this, we first analyze the write procedure of JFFS2 filesystem in detail, and derive the drawback and overhead. Then, we design the flash write back routine for deferred I/O. We apply it to the Linux JFFS2 by implementing fflush and flash_writeback kernel thread. The designed flash write back routine can reduce average write latency when the kernel buffers are enough to get the users data

Data placement and prefetching with accurate bit rate control for interactive media server

An interactive Media Server should support unrestricted control to viewers with their service level agreements. It is important to manage video data effectively to facilitate efficient retrieval. In this paper, we propose an efficient placement algorithm as part of an effective retrieval scheme to increase the number of clients who can be provided with interactive service. The proposed management schemes are incorporated with a bit count control method that is based on repeated tuning of quantization parameters to adjust the actual bit count to the target bit count. The encoder using this method can generate coded frames whose sizes are synchronized with the RAID stripe size, so that when various fast-forward levels are accessed we can reduce the seek and rotational latency and enhance the disk throughput of each disk in the RAID system. Experimental results demonstrate that the proposed schemes can significantly improve the average service time and guarantee more users service of quality, and the interactive media server can thereby efficiently service a large number of clients.

Scrap: Data Reorganization and Placement of Two Dimensional Scalable Video in a Disk Array-Based Video Server

In this paper, we propose an efficient data reorgani- zation and placement for two dimensional scalable video in a disk array-based video server. In the scalable video coding (SVC), multi-dimensional scalability aspects are implemented, accordingly, all types of these scalability can be exploited at the same time. However, its flexibility would make a non contiguous retrieval of partial stream data, which degrade disk performance greatly. We note that in a disk array the video data can be split into several discontinuous sub-streams by the striping manner of disk array. In view of this, we reorganize sub-streams taking into account both of the decoding dependency of two dimensional scalable video and the location in a disk array. It is shown that the advantages of our placement include: minimizing the seek frequency while maximizing utilization of each disk, maximizing the load balancing and parallelism of disk array. The experimental results show that the proposed reorganization and placement of two dimensional scalable video can significantly improve the performance of video server. keywords - Sclable Video Coding, Unit Sub-stream, Disk Array