###### Abstract  
In this paper, we propose an active file mode change mechanism in which the file synchronization system of cloud storage automatically changes files in a directory of a client to the online or local mode by considering tradeoff between local storage usage and file access time according to directory activation ratio. When the directory activation ratio rises above a certain threshold, the proposed active file mode change mechanism selects online mode files in this directory based on file access delay time and local storage usage and changes them to the local mode to reduce file access delay of active IoT clients. When the directory activation ratio falls below the threshold, the active file mode change mechanism selects the local mode files based on the last access time and local storage usage and changes them to the online mode to increase available local storage. Experimental results show that the proposed active file mode change mechanism can control when and how much the client can reduce and increase the local storage usage and the file access delay by changing file mode parameters according to the requirements of various IoT devices. active file mode change; directory activation ratio; file synchronization; local mode file; online mode file +
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Existing research on file synchronization [1; 2; 3; 4; 5; 6; 7] focuses on methods to improve synchronization performance to efficiently process large files for many users, but do not support online and local mode change functions for synchronized files. Some commercial cloud file synchronization services [8; 9; 10; 11] provide the functionality to change the online and local modes of files, but this function only allows users to manually change the mode of selected files on an on-demand basis. There are also cloud storage and distributed file systems [12; 13; 14; 15; 16; 17; 18; 19; 20] and other multi-tiered storage systems [21; 22; 23; 24; 25; 26; 27; 28; 29; 30] that use similar approaches to the proposed file mode transition mechanism by prefetching or caching files among storages. However, as they focus more on the performance of file access in costly high-layered storages, cache hit ratio or prediction accuracy are the most important performance metric. In this paper, we propose an active file mode change mechanism that actively changes the files selected in the directory by the file synchronization system to online or local mode according to the directory activation ratio. The active file mode change mechanism first defines the directory activation ratio, which quantifies how active the files in the directory are. If a file is accessed within a reference period, the file activation ratio is 1, which is the maximum value. If the file has not been accessed for a certain period, the file activation ratio starts to drop in proportion to the period. The directory activation ratio is the average value of the activation ratio of all files in the directory. Files in a directory with a low activation ratio waste local storage. The active file mode change mechanism periodically monitors the directory activation ratio, and when this ratio falls below a certain threshold, it selects among the files in the directory and changes them to the online mode in order to increase the available space. The files to be changed to the online mode are selected among the local mode files in consideration of the local storage usage ratio and the most recent access time. Conversely, the more frequently files in the directory are accessed, the higher the directory activation ratio. The active file mode change mechanism selects the online mode files in the directory and changes them to the local mode when the directory activation ratio is higher than a certain threshold. Online mode files to be changed to the local mode are selected in consideration of the local storage usage ratio and file access delay time.