利用遠端儲存日誌來提昇日誌式檔案系統之效能

Abstract

日誌式檔案系統是一個常用的用來增加檔案系統效能與可靠性的技術，目前有很多檔案系統採用這樣的技術，然而採用這種技術的代價是需要額外的空間來儲存檔案系統日誌。此外，因為紀錄日誌引起的額外的磁碟活動也會導致效能降低。根據這篇論文的實驗顯示，不同周期的儲存日誌活動會導致不同的效能降低，最多可達27%左右。

在這一篇論文中，我們利用遠端機器透過網路來儲存日誌的方法，以提升因額外的磁碟活動而導致的效能低落。因為網路傳送資料的速度較快且不影響檔案系統之效能，幾乎可以達到極低週期之儲存日誌活動，這提供了幾乎相同的檔案系統一致性(file system consistency)保證，以及相對於傳統日誌式檔案系統效能提昇。

在實驗結果中，利用遠端儲存檔案系統日誌的方法，可以提升檔案系統效能大約10%~21%左右。對100Mb/s的Ethernet而言，每秒佔用的網路頻寬大約是0.8%到5%左右，而檔案系統的回復速度則與傳統的日誌式檔案系統差不多。

Metadata journaling is a matured technique for improving performance and reducing consistency checking time of file systems. Many file system adopts this scheme for improving performance and file system consistency. However, the overhead brings by metadata journaling is extra space and performance degrade caused by frequent journal data flush. According our experiment, different commit interval and journal mode result in different performance degrade to 27%.

In this paper, we improve performance of journal file system by remote journal scheme. The remote journal scheme removes the frequent journal I/O activities form local disk to remote server. Because network transfer is faster and will not effect file system performance, we can have better throughput and the same consistency semantic.

According to the experiments in this paper, remote journal increases about 10% to 21% performance, but the penalty is light. Although remote journal does need more CPU time for network transfer, the overhead is less than 10%. And the extra network bandwidth taken by remote journal is less than 7% in metadata bound workload.