diff --git a/contrib/perf.py b/contrib/perf.py
--- a/contrib/perf.py
+++ b/contrib/perf.py
@@ -980,9 +980,10 @@ def perffncacheload(ui, repo, **opts):
 def perffncachewrite(ui, repo, **opts):
     timer, fm = gettimer(ui, opts)
     s = repo.store
+    lock = repo.lock()
     s.fncache._load()
-    lock = repo.lock()
     tr = repo.transaction('perffncachewrite')
+    tr.addbackup('fncache')
     def d():
         s.fncache._dirty = True
         s.fncache.write(tr)
diff --git a/tests/test-contrib-perf.t b/tests/test-contrib-perf.t
--- a/tests/test-contrib-perf.t
+++ b/tests/test-contrib-perf.t
@@ -146,7 +146,11 @@ perfstatus
 #if repofncache
   $ hg perffncacheencode
   $ hg perffncacheload
+  $ hg debugrebuildfncache
+  fncache already up to date
   $ hg perffncachewrite
+  $ hg debugrebuildfncache
+  fncache already up to date
 #endif
   $ hg perfheads
   $ hg perfindex