## 6 Performance Evaluation
### Directory Deactivation Experiment
Figures 9-11 show how the local storage usage changes according to various thresholds of the active file mode change mechanism in the directory deactivation scenario. Unlike the manual file mode change mechanism, in all three graphs, we can see that the storage usage decreases after a certain point in time. There is also a characteristic that the timing or rate of decrease in storage usage is different depending on the type and value of the thresholds. In Figures 9 and 10, the storage usage starts to decrease at a later time, whenever the two thresholds of the _DSLAT_ and _OMT_ are increased, respectively. Because the _USRT_ default value is 0.5, storage usage is reduced only to 5 MB, which is 50% of the total amount of storage (10 MB). Figure 9 (_DSLAT_) and Figure 10 (_OMT_) are similar in appearance, but the roles of the two thresholds are different. _DSLAT_ is how long a file can remain active even if it is not accessed. The larger the _DSLAT_ value, the slower the inactivation starts without accessing the file, and the slower the directory activation rate decreases. Since the directory activation ratio is an index used by the active file mode change mechanism to determine whether to change the file mode or not, if the time at which the directory activation ratio decreases is delayed, the time to change the file mode to the online mode is also delayed. That is, in the directory deactivation scenario, the higher the _DSLAT_ value, the slower the time at which the active file mode change mechanism determines that the file mode needs to be changed to the online mode. As a result, as shown in Figure 9, the higher the _DSLAT_ value, the slower the file is changed to online mode (the local storage usage decreases). For example, when the _DSLAT_ is 6 h (dslat-6), the online mode change starts after about 47 h, whereas when the _DSLAT_ is 36 h (dslat-36), the online mode change starts only after 140 h of the experiment. As shown in Figure 10, the _OMT_ also affects when the local storage usage decreases, that is, when files are changed to the online mode. If the directory activation ratio is less than the _OMT_, the active file mode change mechanism determines that the directory is inactive and starts changing files to the online mode. Therefore, as the _OMT_ is smaller, the directory activation ratio must be smaller than this value for the active file mode change mechanism to determine that the directory is inactive. That is, the online mode change operation starts late. For example, if the _OMT_ is 0 (omt-0), the online mode change operation does not start until the end of the experiment because the directory activation ratio cannot be less than 0. If the _OMT_ is 1 (omt-1), the online mode change starts after 47 h, when the directory activation ratio becomes less than 1. At this point, since the addition of new files and the change of existing files to the online mode are performed simultaneously, we can see that the local storage usage maintains the same value, and then increases and decreases repeatedly. To summarize the difference between the roles of the _OMT_ and the  
Figure 8: Change of local storage usage of manual file mode change mechanism in directory deactivation scenario. Figure 11 shows the change in the local storage usage according to _USRT_. The _USRT_ plays a role in limiting the scope of file mode change from the point of view of local storage. In the directory deactivation experiment, when the active file mode change mechanism changes files to the online mode to free up local storage space, it changes to the online mode until the local storage usage ratio does not go below the _USRT_. A high _USRT_ value means that the synchronization system can use a lot of space on local storage; therefore, even if the directory activation ratio is low, it will not make much storage available. For example, if the _USRT_ is 0.8, files are changed to the online mode within the range where the local storage usage ratio is not less than 80%.