Speed Up Syncthing File Sync Discovery (From 11 Seconds to 2)

New files were taking 11 seconds to sync from my Linux box to my Mac. That’s an eternity when you’re iterating on code or config. The fix took 30 seconds.

The Problem

Syncthing uses filesystem watchers to detect changes. When a file changes, the watcher notices, Syncthing scans it, and sync begins. But there’s a built-in delay before Syncthing acts on watcher events.

The default fsWatcherDelayS is 10 seconds. This batches multiple rapid changes into one scan, which makes sense for large codebases. But for small, frequent syncs, it’s pure latency.

The Fix

Set fsWatcherDelayS="1" on the sending machine. The receiver’s setting doesn’t matter for detection speed.

Find your config file:

• Linux: ~/.local/state/syncthing/config.xml or ~/.config/syncthing/config.xml
• macOS: ~/Library/Application Support/Syncthing/config.xml

Find your folder element and change the delay:

<folder id="your-folder-id" label="Sync" path="/path/to/folder" ...>
    <!-- Change this from 10 to 1 -->
    <fsWatcherDelayS>1</fsWatcherDelayS>
    ...
</folder>

Restart Syncthing after editing.

Benchmarks

I measured round-trip detection time: create a file, poll for its appearance on the other machine, record the elapsed time.

# On sender
echo "test" > ~/Sync/test-$(date +%s).txt

# On receiver (running in loop)
while [ ! -f ~/Sync/test-*.txt ]; do sleep 0.1; done

Before (default fsWatcherDelayS=10 on Linux):

Mac already had the delay set to 1 from earlier tinkering, which explains the asymmetry.

After (both set to fsWatcherDelayS=1):

That’s a 4x improvement on the slow path.

Why the Asymmetry?

Linux uses inotify, which is fast and efficient. macOS uses FSEvents, which has higher latency. Even with identical settings, Mac detection will be slightly slower.

The remaining ~2 seconds includes:

• Filesystem watcher delay (1 second)
• Syncthing’s internal scan
• Network round-trip for sync protocol
• File write on receiver

Other Tuning Options

If you’re still hungry for speed:

• pullerMaxPendingKiB: Increase for better throughput on large files (default 128 KiB is conservative)
• disableFsync: Skip fsync on writes for faster file creation. Risky if power fails mid-sync.
• Syncthing 2.0: Uses SQLite instead of flat files for the index database, plus multiple connections per device. Worth upgrading if you haven’t.

Trade-offs

Setting fsWatcherDelayS=1 means more frequent scans. On a folder with thousands of rapid changes (like a build directory), this could increase CPU usage. For typical sync folders with occasional changes, the overhead is negligible.

Takeaways

• The sending machine’s fsWatcherDelayS controls detection speed
• Default of 10 seconds is conservative; 1 second works fine for most use cases
• Linux inotify is faster than macOS FSEvents
• Edit config.xml directly, restart Syncthing