src/Pure/Admin/build_history.scala

 
       val log_path =
         other_isabelle.isabelle_home_user +
           Build_Log.log_subdir(build_history_date) +
           Build_Log.log_filename(
-            BUILD_HISTORY, build_history_date, build_host, ml_platform, "M" + threads).ext("gz")
+            BUILD_HISTORY, build_history_date, build_host, ml_platform, "M" + threads)
 
       val build_info = Build_Log.Log_File(log_path.base.implode, res.out_lines).parse_build_info()
 
       val meta_info =
         List(Build_Log.Field.build_engine -> BUILD_HISTORY,

               Some("Heap " + session_name + " (" + Value.Long(heap.file.length) + " bytes)")
             else None
           })
 
       Isabelle_System.mkdirs(log_path.dir)
-      File.write_gzip(log_path,
+      File.write_xz(log_path.ext("xz"),
         terminate_lines(
           Build_Log.Log_File.print_props(META_INFO_MARKER, meta_info) :: res.out_lines :::
           ml_statistics.map(Build_Log.Log_File.print_props(Build_Log.ML_STATISTICS_MARKER, _)) :::
-          heap_sizes))
+          heap_sizes), XZ.options(6))
 
 
       /* next build */
 
       if (multicore_base && first_build && isabelle_output_log.is_dir)