• JSTS:Journal of Semiconductor Technology and Science
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• v.13 no.4
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• pp.291-302
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• 2013

NAND flash memory (NFM) based storage devices, e.g. Solid State Drive (SSD), are rapidly replacing conventional storage devices, e.g. Hard Disk Drive (HDD). As NAND flash memory technology advances, its specification has evolved to support denser cells and larger pages and blocks. However, efforts to fully understand their impacts on design objectives such as performance, power, and cost for various applications are often neglected. Our research shows this recent trend can adversely affect the design objectives depending on the characteristics of applications. Past works mostly focused on improving the specific design objectives of NFM based systems via various architectural solutions when the specification of NFM is given. Several other works attempted to model and characterize NFM but did not access the system-level impacts of individual parameters. To the best of our knowledge, this paper is the first work that considers the specification of NFM as the design parameters of NAND flash storage devices (NFSDs) and analyzes the characteristics of various synthesized and real traces and their interaction with design parameters. Our research shows that optimizing design parameters depends heavily on the characteristics of applications. The main contribution of this research is to understand the effects of low-level specifications of NFM, e.g. cell type, page size, and block size, on system-level metrics such as performance, cost, and power consumption in various applications with different characteristics, e.g. request length, update ratios, read-and-modify ratios. Experimental results show that the optimized page and block size can achieve up to 15 times better performance than the conventional NFM configuration in various applications. The results can be used to optimize the system-level objectives of a system with specific applications, e.g. embedded systems with NFM chips, or predict the future direction of NFM.

• Spatial Information Research
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• v.20 no.5
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• pp.67-76
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• 2012

Recently the mobile DBMS (Database Management System) is popular to store and manage large data in a mobile device. Especially, the research and development about mobile storage structure and querying method for navigation map data in a mobile device have been performed. The performance of the mobile DBMS in which random data accesses are most queries if the NAND flash memory is used as storage media of the DBMS is degraded. The reason is that the performance of flash memory is good in writing sequentially but bad in writing randomly as the features of the NAND flash memory. So, new storage structure and querying policies of the mobile DBMS are needed in the mobile DBMS in which a flash memory is used as storage media. In this paper, we have studied the policy of the database page management to enhance the performance of the frequent random update and applied this policy to the navigation-specialized mobile DBMS which supports incremental map update. And also we have evaluated the performance of this policy by experiments.

• The Journal of the Korea institute of electronic communication sciences
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• v.18 no.6
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• pp.1299-1306
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• 2023

In this paper, we present a system for undesignated cultural heritage management and real-time theft chase, which uses flash-based large-capacity storage. The proposed system is composed of 3 parts, such as a cultural management device, a flash-based server, and a monitoring service for managing cultural heritages and chasing thefts using IoT technologies. However flash-based storage needs methods to overcome the limited lifespan. Therefore, in this paper, we present a system, which uses the disk buffer in flash-based storage to overcome the disadvantage, and evaluate the system performance in various environments. In our experiments, LRU policy shows the number of direct writes in the flash-based storage by 10.7% on average compared with CLOCK and FCFS.

• Journal of KIISE:Computing Practices and Letters
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• v.16 no.11
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• pp.1031-1040
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• 2010

Hash join is one of the core algorithms in databases management systems. If a hash join cannot complete in one-pass because the available memory is insufficient (i.e., hash table overflow), however, it may incur a few sequential writes and excessive random reads. With harddisk as the tempoary storage for hash joins, the I/O time would be dominated by slow random reads in its probing phase. Meanwhile, flash memory based SSDs (flash SSDs) are becoming popular, and we will witness in the foreseeable future that flash SSDs replace harddisks in enterprise databases. In contrast to harddisk, flash SSD without any mechanical component has fast latency in random reads, and thus it can boost hash join performance. In this paper, we investigate several important and practical issues when flash SSD is used as tempoary storage for hash join. First, we reveal the va patterns of hash join in detail and explain why flash SSD can outperform harddisk by more than an order of magnitude. Second, we present and analyze the impact of cluster size (i.e., va unit in hash join) on performance. Finally, we emperically demonstrate that, while a commerical query optimizer is error-prone in predicting the execution time with harddisk as temporary storage, it can precisely estimate the execution time with flash SSD. In summary, we show that, when used as temporary storage for hash join, flash SSD will provide more reliable cost estimation as well as fast performance.

• Proceedings of the IEEK Conference
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• 2009.05a
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• pp.240-242
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• 2009

Flash memories are one of best media to support future computer's storages. However, we need to improve traditional data management scheme due to the relatively slow characteristics of flash operation of SSD. Due to the unique characteristics of flash media and hard disk, the efficiency of I/O processing is severely reduced without special treatment, especially in the presence of heavy workload or bulk data copy. In this respect, we need to design and develop efficient hybrid-RAID storage system.

• Journal of KIISE:Computer Systems and Theory
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• v.37 no.6
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• pp.330-338
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• 2010

Flash storage devices are very popularly used in portable devices such as cell phones, PDAs and MP3 players. As technology is improved, users want much bigger and faster storage system. Paradoxically, people have to wait more and more time proportionally to the capacity of their storage devices when these are trying to be recovered after file system crash. It is serious problem because booting time of devices is dominated by crash recovery of flash file system. In this paper, we design a crash recovery mechanism, named 'Working Area(WA hereafter)' technique, which has bounded crash recovery execution time. With WA technique, write operations to flash memory are only performed in WA. Therefore, by simply scanning the latest WA. We can recover a file system crash because every change for flash memory is occured only in latest WA. We implement the WA technique based on YAFFS2 and evaluate by comparing with traditional techniques. As a result, WA technique shows that its crash recovery execution time is 25 times faster than Log-based Method when we use 1 gig a bytes NAND flash memory in worst case. This gap will be futher and futher as storage capacity grows.

• Journal of the Institute of Electronics Engineers of Korea CI
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• v.38 no.1
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• pp.26-45
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• 2001

The Flash memory market: is an exciting market that has quickly over the last 10 years. Recently Flash memory provides a high-density. truly non-volatile, high performance read write memory solutions, also is characterized by low power consumption, extreme ruggedness and high reliability. Flash memory is an optimum solution for large nonvolitilc storage operations such as solid file storage, digital video recorder, digital still camera, The MP3 player and other portable multimedia communication applications requiring non-volatility. Regardless of the type of Flash memory, Flash media management software is always required to manage the larger Flash memory block partitions. This is true, since Flash memory cannot be erased on the byte level common to memory, but must be erased on a block granularity. The management of a Flash memory manager requires a keen understanding of a Flash technology and data management methods. Though Flash memory's write performance is relatively slow, the suggested algorithm offers a higher maximum write performance. Algorithms so far developed is not suitable for applications which is requiring more fast and frequent accesses. But, the proposed algorithm is focused on the justifiable operation even in the circumstance of fast and frequent accesses.

• Journal of the Korean Society of Safety
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• v.30 no.4
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• pp.86-91
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• 2015

The flash point is used to categorize inflammable liquids according to their relative flammability. Such a categorization is important for the safe handling, storage, and transportation of inflammable liquids. The flash point temperature of two binary liquid mixtures(n-octane+n-decane and n-octane+n-dodecane) has been measured for the entire concentration range using Seta-flash closed cup tester based on the ASTM D3278 method. The closed cup flash point temperature was estimated using the UNIFAC(Universal Functional Activity Coefficient) group contribution model. The experimentally derived flash point was also compared with the predicted flash point from the UNIFAC model. The UNIFAC model is able to estimate the flash point fairly well for n-octane+n-decane mixture and n-octane+n-dodecane mixture.

• JSTS:Journal of Semiconductor Technology and Science
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• v.15 no.5
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• pp.546-553
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• 2015

Recently, flash cache is widely adopted as the performance accelerator of legacy storage systems. Unlike other cache media, flash cache should be carefully managed as it has peculiar characteristics such as long write latency and limited P/E cycles. In particular, we make two prominent observations that can be utilized in managing flash cache. First, a serious worn-out problem happens when the working-set of a system is beyond the capacity of flash cache due to excessively frequent cache replacement. Second, more than 50% of data has no hit in flash cache as it is a second level cache. Based on these observations, we propose a cache admission control policy that does not cache data when it is first accessed, and inserts it into the cache only after its second access occurs within a certain time window. This allows the filtering of data disruptive to flash cache in terms of endurance and performance. With this policy, we prolong the lifetime of flash cache 2.3 times without any performance degradations.

• Proceedings of the Korea Information Processing Society Conference
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• 2014.11a
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• pp.771-774
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• 2014

지난 몇 년간 스마트폰은 굉장히 빠른 속도로 발전하면서 생활 속에서 큰 비중을 차지하고 있다. 이러한 스마트폰에는 에너지 효율, 크기, 속도 면에서 모바일 기기에 적합한 Flash storage가 탑재되고 있다. 이 논문에서는 스마트폰에 탑재된 Flash storage를 기반으로 한 버퍼 교체 알고리즘들 가운데 Spatial Clock 알고리즘에 초점을 맞추고 있다. 그리고 이 알고리즘이 Video Streaming workload에서 성능 발휘를 하지 못한다는 점을 해결하기 위해 SWSC(Sequential Write Spatial Clock) 알고리즘을 제안하였다. 이 알고리즘은 dirty 페이지들이 연속적인 경우 sequential write를 수행한다. 따라서 write 수행시간을 줄일 수 있고 결과적으로 Video Streaming workload에서도 좋은 성능을 발휘할 수 있다.