I mostly use syncthing to back up and keep my photo library in sync. I’m a Lightroom user, which means I have a “catalog” that contains a database with metadata about photos, and previews/thumbnails etc. This catalog lives on my laptop, but it would be really bad if it got lost or corrupted. The catalog only ever gets changed on my laptop, so that folder is set to “Master” on the laptop.
My actual photos (lots and lots of raw files) live on a file server at home. Losing them would be doubly bad, so apart from local filesystem snapshots and stuff they are synced to another off site file server. All in all that’s three devices, with all three of them sharing one “Lightroom” folder and two of them sharing a “Photos” folder; it could be visualized like this:
+--Laptop | | "Lightroom" folder | | "Photos" folder | | | v | File Server 1 | ^ | | | | "Lightroom" folder | | "Photos" folder | | | v +-->File Server 2
The “Photos” folder is fairly large (~500 GB) and changes only rarely – and then only to add new files, mostly. Hence I’ve set the rescan interval to
86400 (one day, in seconds) to minimize the disk thrashing. The connection between the two file servers is ADSL, less than one Mbps upstream, so delaying things by up to a day is fine anyhow.