diff -r a0ec05d93c8e -r c577bb4a04d4 mercurial/debugcommands.py
--- a/mercurial/debugcommands.py	Mon Feb 10 17:31:05 2020 -0500
+++ b/mercurial/debugcommands.py	Wed Jan 15 15:47:12 2020 +0100
@@ -95,7 +95,10 @@
     stringutil,
 )
 
-from .revlogutils import deltas as deltautil
+from .revlogutils import (
+    deltas as deltautil,
+    nodemap,
+)
 
 release = lockmod.release
 
@@ -2082,6 +2085,20 @@
 
 
 @command(
+    b'debugnodemap',
+    [(b'', b'dump', False, _(b'write persistent binary nodemap on stdin'))],
+)
+def debugnodemap(ui, repo, **opts):
+    """write and inspect on disk nodemap
+    """
+    if opts['dump']:
+        unfi = repo.unfiltered()
+        cl = unfi.changelog
+        data = nodemap.persistent_data(cl.index)
+        ui.write(data)
+
+
+@command(
     b'debugobsolete',
     [
         (b'', b'flags', 0, _(b'markers flag')),