This page mostly duplicates syncthing(1). Needs merging.

Debugging Syncthing

There’s a lot that happens behind the covers, and Syncthing is generally quite silent about it. A number of environment variables can be used to set the logging to verbose for various parts of the program, and to enable profiling.

Environment Variables


The environment variable STTRACE can be set to a comma separated list of “facilities”, to enable extra debugging information for said facility. A facility generally maps to a Go package, although there are a few extra that map to parts of the main package. Currently, the following facilities are supported (an up to date list is always printed by syncthing --help):

  • beacon (Multicast and broadcast discovery)
  • config (Configuration loading and saving)
  • connections (Connection handling)
  • db (The database layer)
  • dialer (Dialing connections)
  • discover (Remote device discovery)
  • events (Event generation and logging)
  • http (REST API)
  • main (Main package)
  • model (The root hub)
  • nat (NAT discovery and port mapping)
  • pmp (NAT-PMP discovery and port mapping)
  • protocol (The BEP protocol)
  • scanner (File change detection and hashing)
  • sha256 (SHA256 hashing package)
  • stats (Persistent device and folder statistics)
  • sync (Mutexes)
  • upgrade (Binary upgrades)
  • upnp (UPnP discovery and port mapping)
  • versioner (File versioning)
  • all (all of the above)

The debug output is often of the kind that it doesn’t make much sense without looking at the code. The purpose of some of the different packages / facilities are something like this:

  • beacon sends and receives UDP broadcasts used by the local discovery system. Debugging here will show which interfaces and addresses are selected for broadcasts, etc.
  • discover sends and receives local discovery packets. Debugging here will output the parsed packets, devices that get registered etc.
  • model is the largest chunk of the system; this is where pulling of out of date files happens, indexes are sent and received, and incoming requests for file chunks are logged.
  • scanner is the local filesystem scanner. Debugging here will output information about changed and unchanged files.

Enabling any of the facilities will also change the log format to include microsecond timestamps and file names plus line numbers. This can be used to enable this extra information on the normal logging level, without enabling any debugging: STTRACE=somethingnonexistent for example.

Under Unix (including Mac) the easiest way to run Syncthing with an environment variable set is to prepend the variable to the command line. I.e:

$ STTRACE=model syncthing

On windows, it needs to be set prior to running Syncthing.

C:\> set STTRACE=model
C:\> syncthing


The STPROFILER environment variable sets the listen address for the HTTP profiler. If set to for example :9090 the profiler will start and listen on port 9090. http://localhost:9090/debug/pprof is then the address to the profiler. Se go tool pprof for more information.


Directory to load GUI assets from. Overrides compiled in assets. Useful for developing webgui, commonly use STGUIASSETS=gui bin/syncthing


Write a CPU profile to cpu-$pid.pprof on exit.


Write heap profiles to heap-$pid-$timestamp.pprof each time heap usage increases.


Write block profiles to block-$pid-$timestamp.pprof every 20 seconds.


Write running performance statistics to perf-$pid.csv. Not supported on Windows.


Disable automatic upgrades.


Set the maximum number of CPU cores to use. Defaults to all available CPU cores.


Percentage of heap growth at which to trigger GC. Default is 100. Lower numbers keep peak memory usage down, at the price of CPU usage (ie. performance)