diff --git a/mercurial/configitems.py b/mercurial/configitems.py
--- a/mercurial/configitems.py
+++ b/mercurial/configitems.py
@@ -681,6 +681,9 @@
     b'experimental', b'exp-persistent-nodemap.mmap', default=True,
 )
 coreconfigitem(
+    b'experimental', b'exp-persistent-nodemap.mode', default=b'compat',
+)
+coreconfigitem(
     b'experimental', b'server.filesdata.recommended-batch-size', default=50000,
 )
 coreconfigitem(
diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
--- a/mercurial/localrepo.py
+++ b/mercurial/localrepo.py
@@ -937,6 +937,8 @@
         options[b'exp-persistent-nodemap'] = True
     if ui.configbool(b'experimental', b'exp-persistent-nodemap.mmap'):
         options[b'exp-persistent-nodemap.mmap'] = True
+    epnm = ui.config(b'experimental', b'exp-persistent-nodemap.mode')
+    options[b'exp-persistent-nodemap.mode'] = epnm
     if ui.configbool(b'devel', b'persistent-nodemap'):
         options[b'devel-force-nodemap'] = True
 
diff --git a/mercurial/revlogutils/nodemap.py b/mercurial/revlogutils/nodemap.py
--- a/mercurial/revlogutils/nodemap.py
+++ b/mercurial/revlogutils/nodemap.py
@@ -13,6 +13,8 @@
 import re
 import struct
 
+from ..i18n import _
+
 from .. import (
     error,
     node as nodemod,
@@ -105,6 +107,9 @@
     def addabort(self, *args, **kwargs):
         pass
 
+    def _report(self, *args):
+        pass
+
 
 def update_persistent_nodemap(revlog):
     """update the persistent nodemap right now
@@ -138,6 +143,11 @@
     ondisk_docket = revlog._nodemap_docket
     feed_data = util.safehasattr(revlog.index, "update_nodemap_data")
     use_mmap = revlog.opener.options.get(b"exp-persistent-nodemap.mmap")
+    mode = revlog.opener.options.get(b"exp-persistent-nodemap.mode")
+    if not can_incremental:
+        msg = _(b"persistent nodemap in strict mode without efficient method")
+        if mode == b'warn':
+            tr._report(b"%s\n" % msg)
 
     data = None
     # first attemp an incremental update of the data
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
@@ -10,7 +10,9 @@
   > [devel]
   > persistent-nodemap=yes
   > EOF
-  $ hg debugbuilddag .+5000 --new-file
+  $ hg debugbuilddag .+5000 --new-file --config "experimental.exp-persistent-nodemap.mode=warn"
+  persistent nodemap in strict mode without efficient method (no-rust no-pure !)
+  persistent nodemap in strict mode without efficient method (no-rust no-pure !)
   $ hg debugnodemap --metadata
   uid: ???????????????? (glob)
   tip-rev: 5000