• Journal of KIISE:Computing Practices and Letters
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• v.16 no.3
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• pp.371-375
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• 2010

SSD is a storage medium based on NAND Flash memory. Because of its short latency, low power consumption, and resistance to shock, it's not only used in PC but also in server computers. Most SSDs use FTL to overcome the erase-before-overwrite characteristic of NAND flash. There are several types of FTL, but page mapped FTL shows better performance than others. But its usefulness is limited because of its large memory footprint for the mapping table. For example, 64MB memory space is required only for the mapping table for a 64GB MLC SSD. In this paper, we propose a novel caching scheme for the mapping table. By using the mapping-table-meta-data we construct a fully associative cache, and translate the address within O(1) time. The simulation results show more than 80 hit ratio with 32KB cache and 90% with 512KB cache. The overall memory footprint was only 1.9% of 64MB. The time overhead of cache miss was measured lower than 2% for most workload.

• Journal of KIISE:Computer Systems and Theory
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• v.37 no.3
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• pp.161-171
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• 2010

As embedded systems adopt monolithic kernels, NAND flash memory is used for swap space of virtual memory systems. While flash memory has the advantages of low-power consumption, shock-resistance and non-volatility, it requires garbage collections due to its erase-before-write characteristic. The efficiency of garbage collection scheme largely affects the performance of flash memory. This paper proposes a novel garbage collection technique which exploits data redundancy between the main memory and flash memory in flash memory-based virtual memory systems. The proposed scheme takes the locality of data into consideration to minimize the garbage collection overhead. Experimental results demonstrate that the proposed garbage collection scheme improves performance by 37% on average compared to previous schemes.

• Journal of KIISE:Computer Systems and Theory
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• v.37 no.2
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• pp.90-102
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• 2010

Flash memory has been widely used as an important storage device for consumer electronics. For the flash memory-based storage systems, FTL (Flash Translation Layer) is used to handle the mapping between a logical page address and a physical page address. Especially, log buffer-based FTLs provide a good performance with small-sized mapping information. In designing the log buffer-based FTL, one important factor is to determine the mapping structure between data blocks and log blocks, called associativity. While previous works use static associativity fixed at the design time, we propose a new log block mapping scheme which adjusts associativity based on the run-time workload. Our proposed scheme improves the I/O performance about 5~16% compared to the static scheme by adjusting the associativity to provide the best performance.

• Journal of the Institute of Electronics Engineers of Korea CI
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• v.46 no.4
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• pp.21-30
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• 2009

Fusion flash memory such as OneNAND$^{TM}$ is popular as a ubiquitous storage device for embedded systems because it has advantages of NAND and NOR flash memory that it can support large capacity, fast read/write performance and XIP(eXecute-In-Place). Besides, OneNAND$^{TM}$ provides not only advantages of hybrid structure but also multi-block erase function that improves slow erase performance by erasing the multiple blocks simultaneously. But traditional NAND Flash Translation Layer may not fully support it because the garbage collection of traditional FTL only considers a few block as victim block and erases them. In this paper, we propose an Erase Group Flash Translation Layer for improving multi-block erase function. EGFTL uses a superblock scheme for enhancing garbage collection performance and invalid block management to erase multiple blocks simultaneously. Also, it uses clustered hash table to improve the address translation performance of the superblock scheme. The experimental results show that the garbage collection performance of EGFTL is 30% higher than those of traditional FTLs, and the address translation performance of EGFTL is 5% higher than that of Superblock scheme.

• Journal of KIISE:Databases
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• v.35 no.5
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• pp.400-410
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• 2008

Flash memory has been widely used in mobile devices, such as mobile phone and digital camera. Recently flash SSD(Solid State Disk), having same interface of the disk drive, is replacing the hard disk of some laptop computers. However, flash memory still cannot be considered as the storage of database systems. The FTL(Flash Translation Layer) of commercial flash SSD, making flash memory operate exactly same as a hard disk, shows poor performance on the workload of databases with many random overwrites. Recently In-Page Logging(IPL) approach was proposed to solve this problem. In this paper, we implement IPL approach on SQLite, a popular open source embedded DBMS, and evaluate its performance. It improves the performance by up to 30 factors for update queries.

• Journal of KIISE
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• v.42 no.9
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• pp.1078-1089
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• 2015

In this paper, we propose the Min-hash Assisted Delta-compression Engine(MADE) to improve the lifetime of NAND flash-based storages at the device level. MADE effectively reduces the write traffic to NAND flash through the use of a novel delta compression scheme. The delta compression performance was optimized by introducing min-hash based LSH(Locality Sensitive Hash) and efficiently combining it with our delta compression method. We also developed a delta encoding technique that has functionality equivalent to deduplication and lossless compression. The results of our experiment show that MADE reduces the amount of data written on NAND flash by up to 90%, which is better than a simple combination of deduplication and lossless compression schemes by 12% on average.

• Journal of KIISE:Databases
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• v.37 no.6
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• pp.292-299
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• 2010

Recently, as flash memory is used as digital storage devices, necessity for digital forensics is growing in a flash memory area for digital evidence analysis. For this purpose, it is important to recover crashed files stored on flash memory efficiently. However, it is inefficient to apply the hard disk based file recovery techniques to flash memory, since hard disk and flash memory have different characteristics, especially flash memory being unable to in-place update. In this paper, we propose a flash-aware file recovery technique for digital forensics. First, we propose an efficient search technique to find all crashed files. This uses meta-data maintained by FTL(Flash Translation Layer) which is responsible for write operation in flash memory. Second, we advise an efficient recovery technique to recover a crashed file which uses data location information of the mapping table in FTL. Through diverse experiments, we show that our file recovery technique outperforms the hard disk based technique.

• Journal of KIISE:Computing Practices and Letters
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• v.15 no.9
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• pp.685-689
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• 2009

Flash memory has several features such as low~power consumption and fast access so that there has been various research on using flash memory as new storage. Especially the Solid State Disk which is composed of flash memory chips has recently replaced the hard disk. At present, SSD adopts the multi-channel and multi-way architecture to exploit advantages of parallel access. In this architecture, data are written on SSD in a unit of a superblock which is composed of multiple blocks in which some blocks are put together. This paper proposes two schemes of selecting, segmenting and re-composing victim superblocks to optimize concurrent processing when a buffer flush occurs. The experimental results show that 35% of superblock- based write operations is reduced by selecting victims and additional 9% by composition of superblock.

• Proceedings of the Korea Information Processing Society Conference
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• 2009.04a
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• pp.699-702
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• 2009

저 전력, 내구성, 소형, 빠른 속도 등의 장점을 가지고 있는 플래시 메모리는 생산 기술 발전에 힘입어 센서 노드, 휴대폰, MP3, PMP등의 소형 전자 제품의 저장장치에서부터 SSD형태로 노트북이나 서버에 이르기 까지 그 활용범위가 더욱 확장되어 가고 있다. 다양한 시스템에서 사용될 수 있는 플래시 메모리의 특성상 이에 저장된 데이터의 호환성은 중요한 고려사항이다. 이를 위해 플래시 메모리의 고유한 특성을 숨기고 일반적인 블록장치로 에뮬레이션 해주는 소프트웨어인 FTL과 FAT 파일시스템이 플래시 메모리 관리를 위한 사실상 표준 소프트웨어로써 사용되고 있다. 그러나 범용 컴퓨터를 기반으로 개발된 FTL과 FAT 파일시스템을 열악한 하드웨어로 구성된 시스템에서 구동하는 경우 많은 제약이 발생한다. 따라서 본 논문에서는 이러한 제약사항을 극복하기 위해 최소한의 파일시스템 기능을 제공하는 FAT 표준 호환 FTL을 제안한다. 제안된 기법은 리눅스 운영체제에 동적으로 적재 가능한 모듈형태로 구현되었으며, 실험을 통해 본 논문에서 제안한 기법이 기존 기법 대비 32%의 메모리 공간을 절약할 수 있으며, 동시에 완벽한 FAT 호환성을 제공함을 확인할 수 있었다.

• Proceedings of the Korea Information Processing Society Conference
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• 2007.11a
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• pp.755-758
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• 2007

플래시 메모리는 휴대폰, MP3 플레이어, 개인휴대정보단말기(PDA), 휴대용 멀티미디어 플레이어(PMP), 디지털 카메라 및 캠코더와 같은 이동성이 강한 소형기기에서 가장 많이 사용되는 저장 매체이다. 최근 대용량의 값싼 플래시 메모리가 등장하면서 랩톱이나 데스크톱과 같은 일반적인 컴퓨팅 환경을 지닌 기기들에서도 그 사용이 확대되고 있는 추세이다. 플래시 메모리가 보다 범용적인 저장 장치로 사용되기 위해서는 일반 컴퓨팅 환경에서의 복잡한 작업 부하에서도 우수한 성능을 제공할 수 있는 플래시 변환 계층(Flash Translation Layer)이 반드시 필요하다. 아쉽게도 현재까지 연구된 FTL 기법들은 소형기기의 단순한 작업 부하에 알맞도록 설계되어 있으며, 일반 컴퓨팅 환경과 같이 복잡한 작업 부하를 지닌 환경에서는 우수한 성능을 제공하지 못한다는 단점을 가지고 있다. 본 논문에서는 일반 컴퓨팅 환경의 복잡한 작업 부하에 대해서도 우수한 가비지 수집 성능을 제공하는 새로운 로그 블록 교체 기법을 제안하였다. 실험을 통한 평가 결과, 제안한 기법은 기존 기법 대비 평균 35% 정도의 가비지 수집 부하를 감소시키는 것으로 나타났다.