diff --git a/docs/check_running.py b/docs/check_running.py
new file mode 100644
index 000000000..ecc55da34
--- /dev/null
+++ b/docs/check_running.py
@@ -0,0 +1,47 @@
+
+import psutil
+import filelock
+
+
+def can_spawn_tahoe(pidfile):
+    """
+    Determine if we can spawn a Tahoe-LAFS for the given pidfile. That
+    pidfile may be deleted if it is stale.
+
+    :param pathlib.Path pidfile: the file to check, that is the Path
+        to "running.process" in a Tahoe-LAFS configuration directory
+
+    :returns bool: True if we can spawn `tahoe run` here
+    """
+    lockpath = pidfile.parent / (pidfile.name + ".lock")
+    with filelock.FileLock(lockpath):
+        try:
+            with pidfile.open("r") as f:
+                pid, create_time = f.read().strip().split(" ", 1)
+        except FileNotFoundError:
+            return True
+
+    # somewhat interesting: we have a pidfile
+    pid = int(pid)
+    create_time = float(create_time)
+
+    try:
+        proc = psutil.Process(pid)
+        # most interesting case: there _is_ a process running at the
+        # recorded PID -- but did it just happen to get that PID, or
+        # is it the very same one that wrote the file?
+        if create_time == proc.create_time():
+            # _not_ stale! another intance is still running against
+            # this configuration
+            return False
+
+    except psutil.NoSuchProcess:
+        pass
+
+    # the file is stale
+    pidfile.unlink()
+    return True
+
+
+from pathlib import Path
+print("can spawn?", can_spawn_tahoe(Path("running.process")))
diff --git a/docs/running.rst b/docs/running.rst
index 406c8200b..2cff59928 100644
--- a/docs/running.rst
+++ b/docs/running.rst
@@ -124,6 +124,35 @@ Tahoe-LAFS.
 .. _magic wormhole: https://magic-wormhole.io/
 
 
+Multiple Instances
+------------------
+
+Running multiple instances against the same configuration directory isn't supported.
+This will lead to undefined behavior and could corrupt the configuration state.
+
+We attempt to avoid this situation with a "pidfile"-style file in the config directory called ``running.process``.
+There may be a parallel file called ``running.process.lock`` in existence.
+
+The ``.lock`` file exists to make sure only one process modifies ``running.process`` at once.
+The lock file is managed by the `lockfile <https://pypi.org/project/lockfile/>`_ library.
+If you wish to make use of ``running.process`` for any reason you should also lock it and follow the semantics of lockfile.
+
+If ``running.process` exists it file contains the PID and the creation-time of the process.
+When no such file exists, there is no other process running on this configuration.
+If there is a ``running.process`` file, it may be a leftover file or it may indicate that another process is running against this config.
+To tell the difference, determine if the PID in the file exists currently.
+If it does, check the creation-time of the process versus the one in the file.
+If these match, there is another process currently running.
+Otherwise, the file is stale -- it should be removed before starting Tahoe-LAFS.
+
+Some example Python code to check the above situations:
+
+.. literalinclude:: check_running.py
+
+
+
+
+
 A note about small grids
 ------------------------