message: image: don't sort prefetch files by their sizes
  Don't try to sort readahead files by their sizes,  thus to keep files
  belonging to the same directory arranged in adjacent is blob file. Together with
  BFS style collecting descendants inodes, it will have a higher merging possibility.
diff: @@ -386,13 +386,14 @@ impl Builder {
  /// Dump blob file and generate chunks
  fn dump_blob(&mut self) -> Result<(Sha256, usize, usize)> {
  - // Sort readahead list by file size for better prefetch
  - let mut readahead_files = self
  + // NOTE: Don't try to sort readahead files by their sizes, thus to keep files
  + // belonging to the same directory arranged in adjacent in blob file. Together with
  + // BFS style collecting descendants inodes, it will have a higher merging possibility.
  + let readahead_files = self
  .readahead_files
  .values()
  .filter_map(|index| index.as_ref())
  .collect::<Vec<&u64>>();
  - readahead_files.sort_by_key(|index| self.nodes[**index as usize - 1].inode.i_size);
  let blob_index = self.blob_table.entries.len() as u32;
  