diff --git a/mercurial/transaction.py b/mercurial/transaction.py
--- a/mercurial/transaction.py
+++ b/mercurial/transaction.py
@@ -405,6 +405,7 @@ class transaction(object):
                     self.report("couldn't remote %s: %s\n"
                                 % (vfs.join(b), inst))
         self.entries = []
+        self._writeundo()
         if self.after:
             self.after()
         if self.opener.isfile(self.journal):
@@ -440,6 +441,32 @@ class transaction(object):
         scope)'''
         self._abort()
 
+    def _writeundo(self):
+        """write transaction data for possible future undo call"""
+        if self.undoname is None:
+            return
+        undobackupfile = self.opener.open("%s.backupfiles" % self.undoname, 'w')
+        undobackupfile.write('%d\n' % version)
+        for l, f, b, c in self._backupentries:
+            if not f:  # temporary file
+                continue
+            if not b:
+                u = ''
+            else:
+                if l not in self._vfsmap and c:
+                    self.report("couldn't remote %s: unknown cache location"
+                                "%s\n" % (b, l))
+                    continue
+                vfs = self._vfsmap[l]
+                base, name = vfs.split(b)
+                assert name.startswith(self.journal), name
+                uname = name.replace(self.journal, self.undoname, 1)
+                u = vfs.reljoin(base, uname)
+                util.copyfile(vfs.join(b), vfs.join(u), hardlink=True)
+            undobackupfile.write("%s\0%s\0%s\0%d\n" % (l, f, u, c))
+        undobackupfile.close()
+
+
     def _abort(self):
         self.count = 0
         self.usages = 0
diff --git a/tests/test-fncache.t b/tests/test-fncache.t
--- a/tests/test-fncache.t
+++ b/tests/test-fncache.t
@@ -81,6 +81,7 @@ Non store repo:
   .hg/phaseroots
   .hg/requires
   .hg/undo
+  .hg/undo.backupfiles
   .hg/undo.bookmarks
   .hg/undo.branch
   .hg/undo.desc
@@ -114,6 +115,7 @@ Non fncache repo:
   .hg/store/data/tst.d.hg/_foo.i
   .hg/store/phaseroots
   .hg/store/undo
+  .hg/store/undo.backupfiles
   .hg/store/undo.phaseroots
   .hg/undo.bookmarks
   .hg/undo.branch
diff --git a/tests/test-hardlinks.t b/tests/test-hardlinks.t
--- a/tests/test-hardlinks.t
+++ b/tests/test-hardlinks.t
@@ -50,6 +50,8 @@ Prepare repo r1:
   1 r1/.hg/store/fncache
   1 r1/.hg/store/phaseroots
   1 r1/.hg/store/undo
+  1 r1/.hg/store/undo.backup.fncache
+  1 r1/.hg/store/undo.backupfiles
   1 r1/.hg/store/undo.phaseroots
 
 
@@ -80,6 +82,8 @@ Repos r1 and r2 should now contain hardl
   2 r1/.hg/store/fncache
   1 r1/.hg/store/phaseroots
   1 r1/.hg/store/undo
+  1 r1/.hg/store/undo.backup.fncache
+  1 r1/.hg/store/undo.backupfiles
   1 r1/.hg/store/undo.phaseroots
 
   $ nlinksdir r2/.hg/store
@@ -99,6 +103,7 @@ Repo r3 should not be hardlinked:
   1 r3/.hg/store/fncache
   1 r3/.hg/store/phaseroots
   1 r3/.hg/store/undo
+  1 r3/.hg/store/undo.backupfiles
   1 r3/.hg/store/undo.phaseroots
 
 
@@ -124,6 +129,9 @@ Create a non-inlined filelog in r3:
   1 r3/.hg/store/fncache
   1 r3/.hg/store/phaseroots
   1 r3/.hg/store/undo
+  1 r3/.hg/store/undo.backup.fncache
+  1 r3/.hg/store/undo.backup.phaseroots
+  1 r3/.hg/store/undo.backupfiles
   1 r3/.hg/store/undo.phaseroots
 
 Push to repo r1 should break up most hardlinks in r2:
@@ -151,7 +159,7 @@ Push to repo r1 should break up most har
   1 r2/.hg/store/00manifest.i
   1 r2/.hg/store/data/d1/f2.i
   2 r2/.hg/store/data/f1.i
-  1 r2/.hg/store/fncache
+  2 r2/.hg/store/fncache
 
   $ hg -R r2 verify
   checking changesets
@@ -176,7 +184,7 @@ Committing a change to f1 in r1 must bre
   1 r2/.hg/store/00manifest.i
   1 r2/.hg/store/data/d1/f2.i
   1 r2/.hg/store/data/f1.i
-  1 r2/.hg/store/fncache
+  2 r2/.hg/store/fncache
 
 
   $ cd r3
@@ -210,6 +218,9 @@ r4 has hardlinks in the working dir (not
   2 r4/.hg/store/fncache
   2 r4/.hg/store/phaseroots
   2 r4/.hg/store/undo
+  2 r4/.hg/store/undo.backup.fncache
+  2 r4/.hg/store/undo.backup.phaseroots
+  2 r4/.hg/store/undo.backupfiles
   2 r4/.hg/store/undo.phaseroots
   2 r4/.hg/undo.bookmarks
   2 r4/.hg/undo.branch
@@ -242,6 +253,9 @@ Update back to revision 11 in r4 should 
   2 r4/.hg/store/fncache
   2 r4/.hg/store/phaseroots
   2 r4/.hg/store/undo
+  2 r4/.hg/store/undo.backup.fncache
+  2 r4/.hg/store/undo.backup.phaseroots
+  2 r4/.hg/store/undo.backupfiles
   2 r4/.hg/store/undo.phaseroots
   2 r4/.hg/undo.bookmarks
   2 r4/.hg/undo.branch
diff --git a/tests/test-hook.t b/tests/test-hook.t
--- a/tests/test-hook.t
+++ b/tests/test-hook.t
@@ -158,6 +158,8 @@ more there after
   journal.phaseroots
   phaseroots
   undo
+  undo.backup.fncache
+  undo.backupfiles
   undo.phaseroots
 
 
diff --git a/tests/test-inherit-mode.t b/tests/test-inherit-mode.t
--- a/tests/test-inherit-mode.t
+++ b/tests/test-inherit-mode.t
@@ -81,6 +81,7 @@ new directories are setgid
   00660 ./.hg/store/fncache
   00660 ./.hg/store/phaseroots
   00660 ./.hg/store/undo
+  00660 ./.hg/store/undo.backupfiles
   00660 ./.hg/store/undo.phaseroots
   00660 ./.hg/undo.bookmarks
   00660 ./.hg/undo.branch
@@ -125,6 +126,7 @@ group can still write everything
   00660 ../push/.hg/store/data/foo.i
   00660 ../push/.hg/store/fncache
   00660 ../push/.hg/store/undo
+  00660 ../push/.hg/store/undo.backupfiles
   00660 ../push/.hg/store/undo.phaseroots
   00660 ../push/.hg/undo.bookmarks
   00660 ../push/.hg/undo.branch