diff --git a/mercurial/bundle2.py b/mercurial/bundle2.py
--- a/mercurial/bundle2.py
+++ b/mercurial/bundle2.py
@@ -1217,7 +1217,11 @@ def handlepushkeyreply(op, inpart):
 def handleobsmarker(op, inpart):
     """add a stream of obsmarkers to the repo"""
     tr = op.gettransaction()
-    new = op.repo.obsstore.mergemarkers(tr, inpart.read())
+    markerdata = inpart.read()
+    if op.ui.config('experimental', 'obsmarkers-exchange-debug', False):
+        op.ui.write(('obsmarker-exchange: %i bytes received\n')
+                    % len(markerdata))
+    new = op.repo.obsstore.mergemarkers(tr, markerdata)
     if new:
         op.repo.ui.status(_('%i new obsolescence markers\n') % new)
     op.records.add('obsmarkers', {'new': new})
diff --git a/tests/test-obsolete.t b/tests/test-obsolete.t
--- a/tests/test-obsolete.t
+++ b/tests/test-obsolete.t
@@ -616,6 +616,15 @@ List of both
 
 #if serve
 
+Test the debug output for exchange
+----------------------------------
+
+  $ hg pull ../tmpb --config 'experimental.obsmarkers-exchange-debug=True' --config 'experimental.bundle2-exp=True'
+  pulling from ../tmpb
+  searching for changes
+  no changes found
+  obsmarker-exchange: 346 bytes received
+
 check hgweb does not explode
 ====================================