Each folder added to a job receives a system .sync folder (hidden by default). It contains share identifier and other service files explained below. Having .sync in the folder is critical for synchronizing. Deleting .sync will result in "Cannot identify destination folder" error. Inside .sync are:
1.The Archive folder stores old versions of files which were either deleted or modified on other devices in the course of syncing. 2. ID file, the identifier of the folder. It lets agents find the right peers and carry out syncing of the same folder added to on other devices. 3. IgnoreList lets you specify which files should be excluded from syncing. 4. StreamsList is a file allowing you to "white list" alternate streams, xattrs and resource forks.
.sync is not created on source agents in Destination and Consolidation jobs.
Could you tell me what the '.!sync' file is in that folder and why it might be growing so large? In this particular case, the folder I'm syncing is just under 1Gb in size (if you exclude the '.!sync' file) with around 5,500 files. The '.!sync' file is around 8.5Gb currently... and syncing to my other devices. I noticed this because the folder was taking a while to sync when it normally syncs very quickly due to only occasional and small changes.
.!sync - is the file that is being downloaded right now. Once done, it's moved out of .sync and placed in its location. If .!sync remains in .sync for too long, it means that syncing stuck for a reason.