• Proceedings of the KSME Conference
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• 2003.11a
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• pp.634-639
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• 2003

It has been studied the flow near a rotating disk with surface topography. The system Ekman number is assumed very small, i.e., $E[{\equiv}\frac{\nu}{{\Omega}^{\ast}L^{\ast2}}]<<1$ in which $L^{\ast}$ denotes a disk radius, ${\nu}$ kinematic viscosity of the fluid and ${\Omega}^{\ast}$ angular velocity of the basic state. Disk surface has a sinusoidal topographic variation along radial coordinate, i.e., $z={\delta}cos(2{\pi}{\omega}r)$, where ${\delta}$ and ${\omega}$ are, respectively, nondimensional amplitude and wave number of the disk surface. Analytic solutions, being useful over the parametric ranges of ${\delta}{\sim}O$( $E^{1/2}$ ) and ${\omega}{\leq}O$ ( $E^{1/2}$ ), are secured in a series-function form of Fourier-Bessel type. An asymptotic behavior, when $E{\rightarrow}0$, is clarified as : for a disk with surface roughness, in contrast to the case of a flat disk, the azimuthal velocity increases in magnitude, together with the thickening boundary layer. The radial velocity, however, decreases in magnitude as the amplitude of surface waviness increases. Consequently, the overall Ekman pumping at the edge of the boundary layer remains unchanged, maintaining the constant value equal to that of the flat disk.

• The KIPS Transactions:PartA
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• v.15A no.5
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• pp.275-286
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• 2008

Fast File System(FFS) utilizes large disk bandwidth to improve the write performance of large files. One way to improve the performance is to write multiple blocks of a large file at a single disk I/O through the disk bandwidth. However, rather than disk bandwidth, the performance of small file writes is limited by disk access times significantly impacted by disk movements such as disk seek and rotation because FFS writes each of small files at a single disk write. We propose CW-FFS (Co-Writing Fast File System) to improve the write performance of small files by minimizing the disk movements that are needed to write small files to disks. Its key technique called co-writing scheme is to dynamically collect multiple small files named by a given directory and then write them at a single disk I/O to contiguous disk locations. Co-writing several small files at a single disk I/O reduces multiple disk movements that are needed for small file writes to one single disk movement, thus increasing the overall write performance of write-intensive applications. Furthermore, a file allocation scheme is introduced to prevent co-writing scheme from having a negative impact on disk spatial locality of small files named by a given directory. The measurement of our technique implemented in the OpenBSD 4.0 shows that CW-FFS increases the performance of small file writes over FFS in the range from 5 to 35% in the Postmark benchmark.

• Journal of Korea Multimedia Society
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• v.13 no.11
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• pp.1594-1609
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• 2010

In the case that multiple services are in race condition for limited I/O resource, the services or processes with lower priority occasionally occupy most of limited I/O resource. And it decreases QoS and performance of important services and makes it difficult to efficiently use limited I/O resource. Although system administrator allocates I/O resource according to priority of process, he/she can't know or expect how much resource will be used by the specific process. Due to these reasons, it causes the problem that he/she can't guarantee the service QoS and performance stability. Therefore, in this paper, we propose service based disk I/O control supporting predictable I/O bandwidth to resolve upper problems. Proposed I/O control guarantees the service QoS and performance stability by supporting the service based predictable I/O bandwidth and it makes limited I/O resource to be efficiently used in respect of service.

• Journal of the Korea Computer Industry Society
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• v.5 no.1
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• pp.57-64
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• 2004

The speed gap between processors and disks is a serious problem. So, I/O sub-system limits the performance of computer system. To overcome the speed gap, caches have been used in computer system. By using cache, the access times to disk blocks can be reduced and the performance of computer system can be improved. In this paper, we proposed an efficient cache management algorithm for computer system which have buffer cache and disk cache. The proposed algorithm can minimize the duplicated blocks between buffet cache and disk cache. We evaluate the proposed algorithm by trace-driven simulation. The simulation results show that the proposed algorithm can reduce the mean access time to disk blocks.

• The Transactions of the Korea Information Processing Society
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• v.7 no.9
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• pp.2827-2835
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• 2000

In recent years, much attention has been focused on improving I/O devices' processing speed which is essential in such large data processing areas as multimedia data processing. And studies on high-performance parallel file systems are considered to be one of such efforts. In this paper, an efficient disk allocation scheme is proposed for high-performance parallel file systems. In other words, the concept of a parallel disk file's parallelism is defined using data declustering characteristic of a given parallel file. With the concept, an efficient disk allocation scheme is proposed which calculates the appropriate degree of data declustering on disks for each parallel file in order to obtain the maximum throughput when more than one parallel file is used at the same time. Since, calculation for obtaining the maximum throughput is too complex as the number of parallel files increases, an approximate disk allocation algorithm is also proposed in this paper. The approximate algorithm is very simple and especially provides very good results when I/O workload is high. In addition, it has shown that the approximate algorithm provides the optimal disk allocation for the maximum throughput when the arrival rate of I/O requests is infinite.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.21 no.11
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• pp.2103-2108
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• 2017

For n points $p_i$ on the m-dimensional plane $R^m$ and a fixed range r, consider a set $T_i$ containing points the distances from $p_i$ of which are less than or equal to r. In case m=1, $T_i$ is an interval on a line, it is a circle on a plane when m=2. For the vertices corresponding to the sets $T_i$, there is an edge between the vertices if the two sets intersect. Then this graph is called an intersection graph G. For m=1 G is called a proper interval graph and for m=2, it is called an unit disk graph. In this paper, we are concerned in the intersection graph G(r) when r changes. In particular, we consider the problem to find the minimum r such that G(r)is connected. For this problem, we propose an O(n) algorithm for the proper interval graph and an $O(n^2{\log}\;n)$ algorithm for the unit disk graph. For the dynamic environment in which the points on a line are added or deleted, we give an O(log n) algorithm for the problem.

• Journal of KIISE:Databases
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• v.31 no.2
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• pp.108-121
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• 2004

In this paper, we propose an enhanced snapshot technique that solves performance degradation when snapshot is initiated for the storage cluster system. However, traditional snapshot technique has some limits adapted to large amount storage shared by multi-hosts in the following aspects. As volume size grows, (1) it deteriorates crucially the performance of write operations due to additional disk access to verify COW is performed. (2) Also it increases excessively the blocking time of write operation performed during the snapshot creation time. (3)Finally, it deteriorates the performance of write operations due to additional disk I/O for mapping block caused by the verification of COW. In this paper, we propose an efficient snapshot technique for large amount storage shared by multi-hosts in SAN Environments. We eliminate the blocking time of write operation caused by freezing while a snapshot creation is performing. Also to improve the performance of write operation when snapshot is taken, we introduce First Allocation Bit(FAB) and Snapshot Status Bit(SSB). It improves performance of write operation by reducing an additional disk access to volume disk for getting snapshot mapping block. We design and implement an efficient snapshot technique, while the snapshot deletion time, improve performance by deallocation of COW data block using SSB of original mapping entry without snapshot mapping entry obtained mapping block read from the shared disk.

• Journal of KIISE:Computer Systems and Theory
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• v.34 no.3
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• pp.101-112
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• 2007

Hard disks have been the most prevalent secondary storage devices and these days their usage is becoming more important in mobile computing systems due to I/O intensive applications such as multimedia applications and games. However, significant power consumption in the disk drives still limits battery lifetimes of mobile systems critically. In this paper, we show that using several smaller disks (instead of one large disk) can be an energy-efficient secondary storage solution on typical mobile platforms without a significant performance delay. Also, we propose a novel energy-efficient technique, which clusters related data into groups and migrates the correlated groups to the same disk. We compare this method with the existing data concentration scheme, and also combine them. The experiments show that our technique saves the energy consumption up to 34% when a pair of 1.8' disks is used instead of a single 2.5' disk with a negligible increase in the average response time. The results also show that our method also saves up to 14.8% of disk energy consumption and improve the average I/O response time by up to 10 times over the existing scheme.

• Journal of KIISE:Computing Practices and Letters
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• v.15 no.5
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• pp.400-404
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• 2009

Native Command Queueing is a technology to maximize throughput of disk system by reordering requests in its command queue. Recent S-ATA 2 standard specifies a protocol for the purpose of dealing with NCQ feature, making most vendors implementing it in their disk devices. However, the new feature, NCQ, may lead to information gap between OS and disk drive. 05 considers that disk will service I/O requests in the order as OS has dispatched. Unfortunately, it isn't true any more since NCQ would simply ignore the policy of OS and reorder the sequence OS has expected. Let us define the term as expectation-discord. Due to the discord, serious performance anomalies or starvation of requests may occur in I/O subsystem. In this paper, we confirm that the expectation-discord actually brings about starvation of requests. We propose a solution to settle it. Our solution is very simple, effective, cheap(not requiring any hardware modification) and portable across various OS. Experimental results show that our solution can balance throughput and response time very well.

• The Bulletin of The Korean Astronomical Society
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• v.40 no.1
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• pp.74.4-75
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• 2015

We investigate the narrow emission-line ratios of 64 radio-loud (log $L_{1.4GHz}$ > 40) AGNs available in the SDSS archive, in order to examine whether there is a systematic difference in the accretion disk condition of radio-loud AGNs compared to radio-quiet AGNs and compact young radio-loud AGNs. The fluxes of narrow-emission lines, [O II], [Ne III], [O III], [O I], [Ar III], are measured for diagnostics. Based on the [O I]/[O III] and [Ar III]/[O III] ratios with photoionization models, we constrain the states of the accretion disk. We will present the results of the emission-line diagnostics.