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標題: 隨選視訊伺服器內之新的資料配置方法
作者: 王宗銘 
關鍵字: 資料配置;擺置樹法;隨選視訊;分散式多媒體
出版社: 國立中興大學工學院;Airiti Press Inc.
Project: 興大工程學刊, Volume 10, Issue 2, Page(s) 47-59.
對於一個以硬碟機為主要儲存機制的隨選視訊系統,視訊資料在硬碟中的資料擺置方法(Data Placement)必須加以嚴謹考量,方能確保資料在儲存時能平均分散,而資料在讀取時能提供穩定的系統效能,以服務多位使用者。先前研究學者所提之各種方法,例如連續資料擺置法、輪流間格法、隨機法等,均各有其缺失;部份方法缺乏彈性,部份則造成系統
效能時好時壞,甚至由於資料結塊之故,導致系統效能普遍低落。因此,本文提出一個新的資料擺置方法,名為擺置樹法(Striping Tree Method)。利用完滿二元樹(Binary Tree)結構上的對稱性,在擺置視訊資料時先尋訪此二元樹,並藉由判斷非終止節點內之旗標值,相互地左
右交換所尋訪之路徑,以達到資料平均分散的目的。在服務多位使用者時,藉由先前尋訪擺置樹時所建構的邏輯檔案表(Logic File Table),系統可正確快速的讀取使用者所需求的視訊資料,提供穩定的系統效能。本方法進一步在吾人所發展的實驗型隨選視訊系統(Victory VoD System)上做實際測試。實驗結果顯示:以此法所發展之磁碟伺服器,在高速乙太網路之傳輸輔助下,單顆硬碟機可以支援至少8位使用者同時上線;系統反應時間其平均值為34ms,此數值與僅服務1位使用者時之系統反應時間相差在1ms以內;以常態分配觀點視之,在95%之機率值內系統反應時間均能保持在20.71ms~48.79ms間。本文之結論為:擺置樹法可以將多部影片資料有效且均勻的分佈於硬碟儲存空間,不至產生資料結塊現象,故能提供穩定之系統效能。

Data placement is a critical problem for system performance in a disk-based VoD system.
Related researchers have presented several methods to tackle this problem including the sequential
data placement, the alternate data placement and the random data placement. However, they all
revealed some drawbacks such as lacking the flexibility, exhibiting remittent performance, or even
deteriorating the system performance due to generating a number of data clots. This paper presents a
new data placement method referred to as the striping tree method. A striping tree, similar to a fully
binary tree, is defined to manipulate the reserve and access of video data on a disk. Within a striping
tree each non-terminal node contains a flag with Boolean values, true or false, and each terminal
node represents a corresponding unit storage space in a disk. Video files are first decomposed into a
serial of video blocks before they are then reserved into a disk in order by traversing the striping tree.
During the tree traversal if the current flag value is true, then the flag value is first set to its opposite
value, and the leftsubtree is continuously being traversed, and vice versa. The traversal is terminated
when a terminal node is being located and a block of video datum being stored. Experiments were
conducted and results were collected on an experimental VoD system referred to as Victory system.
Results demonstrate that when adopting only one hard disk, Victory system is able to support up to 8
users to simultaneously access various movie files. The system access time is 34 ms in average.
Under the assumption that the access time follows the normal distribution with the mean 34 ms and
variance 7.34ms, 95% of the system access time will be in between 20.71~48.79 ms. The concluding
remark is that the striping tree method enables the video data to be evenly distributed in a disk
storage space which in turn facilitates the subsequent data retrieval. Such distribution encourages the
system to perform a near constant system access time for the data retrieval.
ISSN: 1017-4397
Appears in Collections:第10卷 第2期

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