Ridiculous initial sync - it downloads existing data

My setup is dead simple. One directory on a phone, one on a PC. However, the directory is quite big.

I could not change path of a folder on my PC, so decided to remove directory on PC side and create a new one. That attempt failed. So I decided to have a clean setup, recreated config on both sides.

Now, what happened was mind blowing. Syncthing instead of checking if directories are the same by veryfing checksums, syncthing decided to redownload existing data from phone to the PC. Many, many gigs.

Please explain, why is that? Do I have to redownload from my phone all files if I just want to change directory location on my PC?

Syncthing shouldn’t download any actual data if the files already exist. Have you added the folder again at the very same path with the existing data present? How have you determined that the data was downloaded again? The Web GUI should inform you how much actual data has been transferred (screenshots will be welcome too!).

Please also make sure that you’re running Syncthing v1.26.0 on both sides, because with Android involved, I think that https://github.com/syncthing/syncthing/pull/9177 could cause Syncthing to believe that the files on both sides are different, and this issue has just been fixed very recently, with the fix available only in v1.26.0.

PC: v1.25.0 (newest in dnf repository of Fedora) Android: V1.26.0

Steps to reproduce:

  1. Establish two folders with identical content on both sides
  2. Configure Syncthing on both sides.

Expected behaviour:

  1. Syncthing quickly calculates checksums on both sides, and inform users, the directory is up to date.

What happens:

  1. Syncthing on PC is downloading over 10GiB of data.

How I checked it?

  1. Syncthing is informing me about download speed, has progress bar and total data transferred, remaining.
  2. Checked in System manager the traffic (from my phone around 20MiB/s)
  3. Checked with iostat if the data actually gets written on the computer drive. And yes, it was written.

It annoyed me, because for Syncthing it doesn’t matter that the data is already on PC. It could’ve been an empty folder as well - doesn’t matter. And I wouldn’t have such problems in the first place if Syncthing allowed me to change path of a directory on PC side.

The behavior in the scenario you describe here is indeed what you expect: if contents are the same, data is not transferred. I suspect something went wrong in the initial creating/syncing/removing/moving/adding/recreating process you described up top.

Yes, you’re right it was my fault, I put wrong path on PC, and Syncthing created the dir and synced there.

Sorry for disturbing, great piece of soft.


Just for information, you can change the path in the Advanced Configuration (Actions → Advanced). This isn’t the recommended way, but it’s safe as long as you know what you’re doing (e.g. you pause the folder beforehand, then move the data, then unpause under the new path).

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