docs: def fast_deduplication_backup(self, old_backup_entry, process_bar):
          """
          We can just link a old backup entry
  
          :param latest_backup: old BackupEntry model instance
          :param process_bar: tqdm process bar
          """
          # TODO: merge code with parts from deduplication_backup()
          src_path = self.dir_path.resolved_path
          log.debug("*** fast deduplication backup: '%s'", src_path)
          old_file_path = old_backup_entry.get_backup_path()
  
          if not self.path_helper.abs_dst_path.is_dir():
              try:
                  self.path_helper.abs_dst_path.makedirs(mode=phlb_config.default_new_path_mode)
              except OSError as err:
                  raise BackupFileError("Error creating out path: %s" % err)
          else:
              assert not self.path_helper.abs_dst_filepath.is_file(), (
                  "Out file already exists: %r" % self.path_helper.abs_src_filepath
              )
  
          with self.path_helper.abs_dst_hash_filepath.open("w") as hash_file:
              try:
                  old_file_path.link(self.path_helper.abs_dst_filepath)  # call os.link()
              except OSError as err:
                  log.error("Can't link '%s' to '%s': %s" % (old_file_path, self.path_helper.abs_dst_filepath, err))
                  log.info("Mark %r with 'no link source'.", old_backup_entry)
                  old_backup_entry.no_link_source = True
                  old_backup_entry.save()
  
                  # do a normal copy backup
                  self.deduplication_backup(process_bar)
                  return
  
              hash_hexdigest = old_backup_entry.content_info.hash_hexdigest
              hash_file.write(hash_hexdigest)
  
          file_size = self.dir_path.stat.st_size
          if file_size > 0:
              # tqdm will not accept 0 bytes files ;)
              process_bar.update(file_size)
  
          BackupEntry.objects.create(
              backup_run=self.backup_run,
              backup_entry_path=self.path_helper.abs_dst_filepath,
              hash_hexdigest=hash_hexdigest,
          )
  
          if self._SIMULATE_SLOW_SPEED:
              log.error("Slow down speed for tests!")
              time.sleep(self._SIMULATE_SLOW_SPEED)
  
          self.fast_backup = True  # Was a fast backup used?
          self.file_linked = True