diff -r 41e027ab985c -r d389a83b8d55 src/Pure/Admin/build_history.scala
--- a/src/Pure/Admin/build_history.scala	Sun Oct 16 19:18:54 2016 +0200
+++ b/src/Pure/Admin/build_history.scala	Sun Oct 16 20:08:23 2016 +0200
@@ -242,7 +242,7 @@
 
       first_build = false
 
-      (res, log_path)
+      (res, log_path.ext("xz"))
     }
   }