應用於分散式系統的約定式檔案預取技術

Abstract

隨著thin-client/server系統架構不斷改進、分散式檔案系統架構漸趨成熟及網路頻寬的增大，使用者可從任何一台thin-client存取遠端伺服主機所提供的服務，如同使用本機電腦一樣方便。黃冠寰博士提出的MAS TC/S系統是適用於廣域網路的thin-client/server架構，我們根據這個架構思考如何在廣域網路上解決因等候檔案在檔案伺服器與應用伺服器間傳輸所造成的延遲問題。此篇論文提出一種約定式的檔案預取機制來解決此問題。 所謂約定式的檔案預取機制是由使用者運用Appointed File Prefetching Language (AFPL) 去設定預取指令，然後系統根據設定之指令將需要的檔案於適當的時間預取。預取指令包含兩個部份︰選取需要的檔案以及選定預取的方式。藉由從系統及past file access records所提供的檔案資訊，使用者可以選取適當的檔案作為預取的檔案集合，系統運用時間預取或事件預取來執行約定式預取指令︰時間預取是由使用者決定預取檔案集合該在何時傳送至特定應用伺服器的快取內，時間設定可為每日特定的時間、每週特定的時間、或是某個特定的時間。使用者並可設定一行事曆以配合時間預取運作；事件預取是由使用者決定在何種事件觸發後再執行預取指令將預取檔案集合傳送至目前使用者所使用的應用伺服器內，例如當使用者一登入，或是使用某個特定的程式之後。我們的實驗結果證明運用此一方法約可減少30%至90%的時間延遲，並可提高6%至18%的檔案命中率。With the continuous improvements of the technologies of the thin-client/server computing model, the maturity of distributed file system, and the rapid growth of the network bandwidth, a user with a thin-client device can connect to any application server via the network. Hwang et al. purposed the MAS TC/S architecture which targets on the application of thin-client/server computing model on wide area network (WAN). Based on the MAS TC/S architecture, this paper discusses how to reduce the file transformation time between the file server and the application server. In this paper, we propose an appointed file prefetching mechanism to solve this problem. With the appointed file prefetching mechanism, a user or system administrator can write programs which are based on the appointed file prefetching language (AFPL) to instruct the system to prefetch the required files on the specified times. The prefetching instruction on AFPL contains two parts. The first part is used to select the required files and the other is to specify the way for the system to perform prefetching. According to the information in the file system and obtained past file access records, a user can select the needed files as prefetching file set, and s/he can specify to perform time prefetching or event prefetching. The time prefetching is to do prefetching on specific times, like a daily work, a weekly work, or an occasional work. It also can cooperate with the user’s schedule. The event prefetching is to perform prefetching while specific events occur. The events can be the user logins or executing applications and so on. The experimental results show that the time of remote file fetch reduced by 30% to 90% and the hit ratio increased by 6% to 18% in most cases.