While Syncthing 0.14 is running on my main PC and laptop (within LAN), I started renaming subfolders of a synced folder. Each subfolder contains large amount of photos and videos. In total, it was 300GB.
I changed the naming scheme twice so I ended up renaming subfolders twice. Now, while syncing is still happening, I notice my 960GB ssd inside my laptop is suddenly over 750GB full (it was 350GB).
Clearly, during sync, I could see folders with the original name and the new name on my laptop so everything is being copied over. When syncing was finished, folders with the old name were gone.
More proof: On the laptop, .stversions contained hundreds of GB of “deleted” folders and their content, even though the files were renamed server side.
Conclusion: Syncthing did not detect renames and is simply copying entire content over, ending up with double the amount of data (or triple in some cases), which your storage on the receiving end needs to be ready for. This means a huge amount of uneccessary disk I/O and (temporary) storage consumption.
This is on Ubuntu Budgie 20.10, BTRFS filesystem on laptop and MergerFS pool of BTRFS drives on server.
edit: another downside of deleting files in renamed folder (moving them to /.stfolder) and uploading a new version of the file to the new-name folder: although the file is identical to the deleted file, you now store the same file twice. This is why my laptop SSD was filling up.
Is this by design/expected behaviour of Syncthing?