@solf: The USN code will fall back to full scan in the following cases:
- First scan
- A gap in the USN is encountered (this happens when the last backup is too far back, because the USN has a limited size)
- An error / inconsistency has been encountered
- The Duplicati settings have been modified
Furthermore, the algorithm uses the USN only to obtain a list of modified (touched) folders and files. If any folder is found to be modified, its content will be included in a partial scan. So we are not relying on the USN to monitor the exact transactions to files in a modified folder, but do a partial scan.
Your question regarding hard links / junction points is interesting, though. I don’t know how the change journal records files present under several junction points. Under the first junction point? Or under the path the modification was performed. I’ve found this information, which suggests that this is indeed an issue, and it may be wise not to use junction points with the USN, or, as you suggest, force a full-scan every “n” backups.