--- a/mercurial/configitems.py	Fri Feb 14 15:03:26 2020 +0100
+++ b/mercurial/configitems.py	Tue Feb 11 11:18:52 2020 +0100
@@ -666,6 +666,9 @@
     b'experimental', b'exp-persistent-nodemap', default=False,
 )
 coreconfigitem(
+    b'experimental', b'exp-persistent-nodemap.mmap', default=True,
+)
+coreconfigitem(
     b'experimental', b'server.filesdata.recommended-batch-size', default=50000,
 )
 coreconfigitem(