• Journal of KIISE:Computing Practices and Letters
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• v.11 no.4
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• pp.305-311
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• 2005

Flash memory software running on cellular phones and PDAs need to be tested extensively to cope with abrupt power and media faults. For those tests, we designed and implemented a Flash memory emulator with fault injection features. The fault injection tester has provided a helpful framework for designing fault recovery schemes and also for analyzing fault damages to the FTL (Flash Translation Layer) and file system for a Flash memory based system. In this paper, we discuss Plash memory fault types and fault injection features implemented on this Flash memory emulator. We then discuss in detail a design flaw revealed during fault injection tests. Specifically, it was revealed that a scheme that was believed to improve reliability instead, turned out to be harmful. In addition, we discuss post-fault behaviors of the FTL and the file system.

• International Journal of Internet, Broadcasting and Communication
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• v.13 no.2
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• pp.27-35
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• 2021

NAND flash memory-based SSD needs an internal software, Flash Translation Layer(FTL) to provide traditional block device interface to the host because of its physical constraints, such as erase-before-write and large erase block. However, because useful host-side information cannot be delivered to FTL through the narrow block device interface, SSDs suffer from a variety of problems such as increasing garbage collection overhead, large tail-latency, and unpredictable I/O latency. Otherwise, the new type of SSD, open-channel SSD exposes the internal structure of SSD to the host so that underlying NAND flash memory can be managed directly by the host-level FTL. Especially, I/O data classification by using host-side information can achieve the reduction of garbage collection overhead. In this paper, we propose a new scheme to reduce garbage collection overhead of open-channel SSD by separating the journal from other file data for the journaling filesystem. Because journal has different lifespan with other file data, the Write Amplification Factor (WAF) caused by garbage collection can be reduced. The proposed scheme is implemented by modifying the host-level FTL of Linux and evaluated with both Fio and Filebench. According to the experiment results, the proposed scheme improves I/O performance by 46%~50% while reducing the WAF of open-channel SSDs by more than 33% compared to the previous one.

• The KIPS Transactions:PartA
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• v.12A no.7 s.97
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• pp.621-626
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• 2005

Recently, flash memory is widely used in various embedded applications since it has many advantages in terms of non-volatility, fast access speed, shock resistance, and low power consumption. However, it requires a software layer called FTL(Flash Translation Layer) due to its hardware characteristics. We present a new FTL algorithm named LSTAFF(Large State Transition Applied Fast flash Translation Layer) which is designed for large block flash memory The presented LSTAFF is adjusted to flash memory with pages which are larger than operating system data sector sizes and we provide performance results based on our implementation of LSTAFF and previous FTL algorithms using a flash simulator.

• Journal of Digital Contents Society
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• v.14 no.1
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• pp.51-57
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• 2013

Storage device based on Flash Memory have many attractive features such as high performance, low power consumption, shock resistance, and low weight, so they replace HDDs to a certain extent. An Storage device based on Flash Memory has FTL(Flash Translation Layer) which emulate block storage devices like HDDs. A garbage collection, one of major functions of FTL, effects highly on the performance and the lifetime of devices. However, there is no de facto standard for new garbage collection algorithms. To solve this problem, we propose File Clustering Algorithm. File Clustering Algorithm respect to update page from same file at the same time. So, these are clustered to same block. For this mechanism, We propose Page Allocation Policy in FTL and use MIN-MAX GAP to guarantee wear leveling. To verify the algorithm in this paper, we use TPC Benchmark. So, The performance evaluation reveals that the proposed algorithm has comparable result with the existing algorithms(No wear leveling, Hot/Cold) and shows approximately 690% improvement in terms of the wear leveling.

• Journal of KIISE:Computer Systems and Theory
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• v.32 no.9
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• pp.483-490
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• 2005

Recently, flash memory is widely used in embedded applications since it has strong points: non-volatility, fast access speed, shock resistance, and low power consumption. However, due to its hardware characteristics, it requires a software layer called FTL(flash translation layer). The main functionality of FTL is to convert logical addresses from the host to physical addresses of flash memory We present a new FTL algorithm called STAFF(State Transition Applied Fast Flash Translation Layer). Compared to the previous FTL algorithms, STAFF shows five times higher performance than basic block mapping scheme and requires less memory. We provide performance results based on our implementation of STAFF and previous FTL algorithms.

• 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.2
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• pp.77-88
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• 2009

According to the size of NAND flash memory as the storage system of mobile device becomes large, the performance of address translation and life cycle management in FTL (Flash Translation Layer) to interact with file system becomes very important. In this paper, we propose the continuity counters, which represent the number of continuous physical blocks whose logical addresses are consecutive, to reduce the number of address translation. Furthermore we propose the prefetching method which preloads frequently accessed pages into main memory to enhance I/O performance of flash memory. Besides, we use the 2-bit write prediction and asynchronous writing method to predict addresses repeatedly referenced from host and prevent from writing overhead. The experiments show that the proposed method improves the I/O performance and extends the life cycle of flash memory. As a result, proposed CFTL (Clustered Flash Translation Layer)'s performance of address translation is faster 20% than conventional FTLs. Furthermore, CFTL is reduced about 50% writing time than that of conventional FTLs.

• ETRI Journal
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• v.30 no.6
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• pp.790-798
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• 2008

Recently, NAND flash memory has emerged as a next generation storage device because it has several advantages, such as low power consumption, shock resistance, and so on. However, it is necessary to use a flash translation layer (FTL) to intermediate between NAND flash memory and conventional file systems because of the unique hardware characteristics of flash memory. This paper proposes a new clustered FTL (CFTL) that uses clustered hash tables and a two-level software cache technique. The CFTL can anticipate consecutive addresses from the host because the clustered hash table uses the locality of reference in a large address space. It also adaptively switches logical addresses to physical addresses in the flash memory by using block mapping, page mapping, and a two-level software cache technique. Furthermore, anticipatory I/O management using continuity counters and a prefetch scheme enables fast address translation. Experimental results show that the proposed address translation mechanism for CFTL provides better performance in address translation and memory space usage than the well-known NAND FTL (NFTL) and adaptive FTL (AFTL).

• Journal of KIISE
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• v.42 no.4
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• pp.427-433
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• 2015

The rapid advancement of computing technology has triggered the need for fast data I/O processing and high-performance storage technology. Next generation memory technology, which we refer to as new memory, is anticipated to be used for high-performance storage as they have excellent characteristics as a storage device with non-volatility and latency close to DRAM. This research proposes NTL (New memory Translation layer) as a technology to make use of new memory as storage. With the addition of NTL, conventional I/O is served with existing mature disk-based file systems providing compatibility, while new memory I/O is serviced through the NTL to take advantage of the byte-addressability feature of new memory. In this paper, we describe the design of NTL and provide experiment measurement results that show that our design will bring performance benefits.

• Journal of KIISE:Computing Practices and Letters
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• v.13 no.7
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• pp.476-487
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• 2007

Flash memory offers attractive features, such as non-volatile, shock resistance, fast access, and low power consumption for data storage. However, it has one main drawback of requiring an erase before updating the contents. Furthermore, flash memory can only be erased limited number of times. To overcome limitations, flash memory needs a software layer called flash translation layer (FTL). The basic function of FTL is to translate the logical address from the file system like file allocation table (FAT) to the physical address in flash memory. In this paper, a new FTL algorithm called an efficient and advanced space-management technique (EAST) is proposed. EAST improves the performance by optimizing the number of log blocks, by applying the state transition, and by using reallocation blocks. The results of experiments show that EAST outperforms FAST, which is an enhanced log block scheme, particularly when the usage of flash memory is not full.