diff --git a/mercurial/branchmap.py b/mercurial/branchmap.py
--- a/mercurial/branchmap.py
+++ b/mercurial/branchmap.py
@@ -732,7 +732,7 @@ class revbranchcache:
 
         if self._names:
             try:
-                if repo.ui.configbool(b'format', b'mmap-revbranchcache'):
+                if repo.ui.configbool(b'storage', b'revbranchcache.mmap'):
                     with repo.cachevfs(_rbcrevs) as fp:
                         data = util.buffer(util.mmapread(fp))
                 else:
diff --git a/mercurial/configitems.toml b/mercurial/configitems.toml
--- a/mercurial/configitems.toml
+++ b/mercurial/configitems.toml
@@ -2132,6 +2132,11 @@ experimental = true  # experimental as l
 
 [[items]]
 section = "storage"
+name = "revbranchcache.mmap"
+default = true
+
+[[items]]
+section = "storage"
 name = "new-repo-backend"
 default = "revlogv1"
 experimental = true
@@ -2935,8 +2940,3 @@ section = "blackbox"
 name = "date-format"
 default = ""
 in_core_extension = "blackbox"
-
-[[items]]
-section = "format"
-name = "mmap-revbranchcache"
-default = false
diff --git a/tests/test-branches.t b/tests/test-branches.t
--- a/tests/test-branches.t
+++ b/tests/test-branches.t
@@ -2,8 +2,8 @@
 
 #if mmap
   $ cat <<EOF >> $HGRCPATH
-  > [format]
-  > mmap-revbranchcache=true
+  > [storage]
+  > revbranchcache.mmap=true
   > EOF
 #endif