利用混合式快取機制提升行動檔案系統之效能

Abstract

檔案快取機制與策略是決定分散式網路檔案系統傳輸與存取效能的重要因素，尤 其是應用於頻寬受限的行動網路檔案系統中。常見的檔案快取機制有全檔快取 (Whole-File Cache，簡稱WFC)及區塊檔案快取(Block-File Cache，簡稱BFC)。其中， WFC 是一種廣泛應用於分散式網路檔案系統中的檔案快取機制。透過WFC，使用者可 以在本機端保留檔案副本，以節省再次存取相同檔案時所需耗費的網路頻寬與傳輸時 間。BFC 的運作機制則是將檔案切割為許多區塊，並以區塊為單位，進行檔案存取。 在更新檔案內容時，BFC 僅需更新必要的檔案區塊，而不需重傳整個檔案內容。在檔 案內容變動不多的情況下，BFC 可以有效降低檔案更新所需的頻寬需求與傳輸成本。 在這篇論文中，我們設計一個混合式的快取機制（Hybrid Caching Mechanism，簡稱 HCM），結合WFC 與BFC 兩種快取機制， 使其能依據行動檔案系統中的檔案類型、大 小等特性，選擇適用的檔案快取機制，以提升整體行動檔案系統的傳輸與存取效能。 本研究所提出之HCM 能有效避免重覆傳送相同的檔案區塊，並能提供類似串流式檔案 存取功能。在本研究中，我們並透過一連串的實驗，分析並說明引入HCM 後檔案系統 的效能改善。

File caching mechanisms are essential to the performance of network file systems, especially for mobile file systems. The file cache is a specific, reserved area of virtual storage in the range of system memory address. It operates on files or file blocks. Whole-file cache (WFC) is one of the popular file caching mechanisms used in distributed network file systems. With WFC, the file client keeps a local copy of the file obtained from the remote file server. As long as the requested file remains resident in the cache, subsequent file I/O requests are resolved directly from the cache without accessing the remote file server. Hence, the communication costs are reduced. Another file caching mechanism, block-file cache (BFC), intends to store a file in the local cache in blocks. Although more header overheads are introduced to store the block information, only dirty blocks are transmitted during updates if BFC is applied. Compared to WFC, BFC dramatically reduces communication costs if the size of dirty blocks is much smaller than the whole file. In this paper, we design a hybrid caching mechanism (HCM), combining WFC and BFC, to select a proper caching mechanism for a file according to the file type, file size, etc. After conducting a series of experiments, we show that the proposed mechanism offers a better performance than the traditional mobile file systems.