message: nydus-image: set default chunk size to 4M for stargz
  Set default chunk size to 4M for stargz.
diff: @@ -229,7 +229,6 @@ fn prepare_cmd_args(bti_string: String) -> ArgMatches<'static> {
  .long("chunk-size")
  .short("S")
  .help("size of data chunk, must be power of two and between 0x1000-0x1000000:")
  - .default_value("0x100000")
  .required(false)
  .takes_value(true),
  )
  @@ -574,7 +573,6 @@ struct Command {}
  impl Command {
  fn create(matches: &clap::ArgMatches, build_info: &BuildTimeInfo) -> Result<()> {
  let blob_id = Self::get_blob_id(matches)?;
  - let chunk_size = Self::get_chunk_size(matches)?;
  let blob_offset = Self::get_blob_offset(matches)?;
  let parent_bootstrap = Self::get_parent_bootstrap(matches)?;
  let prefetch = Self::get_prefetch(matches)?;
  @@ -585,6 +583,7 @@ impl Command {
  let inline_bootstrap = matches.is_present("inline-bootstrap");
  let repeatable = matches.is_present("repeatable");
  let version = Self::get_fs_version(matches)?;
  + let chunk_size = Self::get_chunk_size(matches, conversion_type)?;
  let aligned_chunk = if version.is_v6() {
  info!("v6 enforces to use \"aligned-chunk\".");
  true
  @@ -1006,9 +1005,15 @@ impl Command {
  Ok(())
  }
  - fn get_chunk_size(matches: &clap::ArgMatches) -> Result<u32> {
  + fn get_chunk_size(matches: &clap::ArgMatches, ty: ConversionType) -> Result<u32> {
  match matches.value_of("chunk-size") {
  - None => Ok(RAFS_DEFAULT_CHUNK_SIZE as u32),
  + None => {
  + if ty == ConversionType::StargzIndexToRef {
  + Ok(0x400000u32)
  + } else {
  + Ok(RAFS_DEFAULT_CHUNK_SIZE as u32)
  + }
  + }
  Some(v) => {
  let param = v.trim_start_matches("0x").trim_end_matches("0X");
  let chunk_size =
  