docs: def fast_compare(self, dir_path):
          """
          :param dir_path: filesystem_walk.DirEntryPath() instance
          """
          if self.latest_backup is None:
              # No old backup run was found
              return
  
          if self.latest_mtime_ns is None:
              # No timestamp from old backup run was found
              return
  
          # There was a completed old backup run
          # Check if we can made a 'fast compare'
          mtime_ns = dir_path.stat.st_mtime_ns
          if mtime_ns > self.latest_mtime_ns:
              # The current source file is newer than
              # the latest file from last completed backup
              log.info("Fast compare: source file is newer than latest backuped file.")
              return
  
          # Look into database and compare mtime and size
  
          try:
              old_backup_entry = BackupEntry.objects.get(
                  backup_run=self.latest_backup,
                  directory__directory=self.path_helper.sub_path,
                  filename__filename=self.path_helper.filename,
                  no_link_source=False,
              )
          except BackupEntry.DoesNotExist:
              log.debug("No old backup entry found")
              return
  
          content_info = old_backup_entry.content_info
  
          file_size = content_info.file_size
          if file_size != dir_path.stat.st_size:
              log.info("Fast compare: File size is different: %i != %i" % (file_size, dir_path.stat.st_size))
              return
  
          old_backup_filepath = old_backup_entry.get_backup_path()
          try:
              old_file_mtime_ns = old_backup_filepath.stat().st_mtime_ns
          except FileNotFoundError as err:
              log.error("Old backup file not found: %s" % err)
              old_backup_entry.no_link_source = True
              old_backup_entry.save()
              return
  
          if old_file_mtime_ns != old_backup_entry.file_mtime_ns:
              log.error("ERROR: mtime from database is different to the file!")
              log.error(" * File: %s" % old_backup_filepath)
              log.error(" * Database mtime: %s" % old_backup_entry.file_mtime_ns)
              log.error(" * File mtime: %s" % old_file_mtime_ns)
  
          if old_file_mtime_ns != dir_path.stat.st_mtime_ns:
              log.info("Fast compare mtime is different between:")
              log.info(" * %s" % old_backup_entry)
              log.info(" * %s" % dir_path)
              log.info(" * mtime: %i != %i" % (old_file_mtime_ns, dir_path.stat.st_mtime_ns))
              return
  
          # We found a old entry with same size and mtime
          return old_backup_entry