The temporary local folder can still help with certain scenarios where nothing else quite fits. Take for instance having a massive file (say a 150 GB one). Now say that it’s in a ‘legacy’ shared folder, with most devices set to use small blocks. Now let’s say you’re expecting each device to change the file just a little bit at some point in the near future. Because of the issues with block sizes, it may be reasonable to reflink the share (or this file) into a temporary local share set to use large blocks. Syncthing will then hash it, and effectively keep both the small block hashes and the large block hashes on hand. So when another device changes the file, the file need not be transferred in full, regardless of whether the device that changed it used small blocks or large ones.
Another scenario is when all sides have the contents of files, but some have inaccurate metadata, and uptime is important. If, say, five devices are all up‐to‐date but two new ones are to join the share, simply adding them would cause lots of metadata changes to take place that you may not want. But by the same token, you might not be able to set the others to send‐only because it is important that they continue to work and synchronize correctly while the new devices are set up. If a device ends up with a broken database and needs to have it be recreated while the share must continue operating for the other devices, such a solution may also be appropriate given the right conditions.
I have scenarios such as these happen occasionally (particularly this past year), and using the temporary local share has been useful, so I just wanted to put it out there as an option. Syncing afresh takes my devices over a month to complete, so any shortcuts to allow the integrity of data and metadata to be preserved while speeding up sychronizing and not sacrificing uptime are important to me.
I’m sad to hear that the enhancement for reflink support won’t help with this scenario. I have situations like this pop up occasionally and I have had devices spend weeks of uptime simply copying files between folders so I really hoped this solution might obviate the need for all that useless physical copying. If the opportunity should arise in the future to detect duplicate files in such a way that they can be reflinked, I would really appreciate it!