diff --git a/mercurial/debugcommands.py b/mercurial/debugcommands.py
--- a/mercurial/debugcommands.py
+++ b/mercurial/debugcommands.py
@@ -2079,16 +2079,29 @@
 
 @command(
     b'debugnodemap',
-    [('', b'dump', False, _(b'write a (binary) serialised nodemap on stdin'))],
+    [
+        (
+            '',
+            b'dump-new',
+            False,
+            _(b'write a (binary) serialised (new) nodemap on stdin'),
+        ),
+        ('', b'dump-disk', False, _(b'dump on-disk data on stdin')),
+    ],
 )
 def debugnodemap(ui, repo, **args):
     """write and inspect on disk nodemap
     """
-    if args['dump']:
+    if args['dump_new']:
         unfi = repo.unfiltered()
         cl = unfi.changelog
         data = nodemap.persistent_data(cl.index)
         ui.write(data)
+    elif args['dump_disk']:
+        unfi = repo.unfiltered()
+        cl = unfi.changelog
+        data = nodemap.persisted_data(cl)
+        ui.write(data)
 
 
 @command(
diff --git a/mercurial/revlogutils/nodemap.py b/mercurial/revlogutils/nodemap.py
--- a/mercurial/revlogutils/nodemap.py
+++ b/mercurial/revlogutils/nodemap.py
@@ -21,6 +21,13 @@
         raise error.RevlogError(b'unknown node: %s' % x)
 
 
+def persisted_data(revlog):
+    """read the nodemap for a revlog from disk"""
+    if revlog.nodemap_file is None:
+        return None
+    return revlog.opener.tryread(revlog.nodemap_file)
+
+
 def setup_persistent_nodemap(tr, revlog):
     """Install whatever is needed transaction side to persist a nodemap on disk
 
diff --git a/tests/test-completion.t b/tests/test-completion.t
--- a/tests/test-completion.t
+++ b/tests/test-completion.t
@@ -290,7 +290,7 @@
   debugmanifestfulltextcache: clear, add
   debugmergestate: 
   debugnamecomplete: 
-  debugnodemap: dump
+  debugnodemap: dump-new, dump-disk
   debugobsolete: flags, record-parents, rev, exclusive, index, delete, date, user, template
   debugp1copies: rev
   debugp2copies: rev
diff --git a/tests/test-persistent-nodemap.t b/tests/test-persistent-nodemap.t
--- a/tests/test-persistent-nodemap.t
+++ b/tests/test-persistent-nodemap.t
@@ -10,9 +10,9 @@
   > exp-persistent-nodemap=yes
   > EOF
   $ hg debugbuilddag .+5000
-  $ hg debugnodemap --dump | f --sha256 --size
+  $ hg debugnodemap --dump-new | f --sha256 --size
   size=245760, sha256=bc400bf49f11e83bbd25630439feee6628a80a8602d2e38972eac44cc3efe10c
-  $ f --sha256 --bytes=256 --hexdump --size < .hg/store/00changelog.n
+  $ hg debugnodemap --dump-disk | f --sha256 --bytes=256 --hexdump --size
   size=245760, sha256=bc400bf49f11e83bbd25630439feee6628a80a8602d2e38972eac44cc3efe10c
   0000: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff |................|
   0010: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff |................|