closer      func() error
	blockSize   int
	streamWg    sync.WaitGroup
	reuseBlocks bool
}

// newMetacacheWriter will create a serializer that will write objects in given order to the output.
// Provide a block size that affects latency. If 0 a default of 128KiB will be used.
// Block size can be up to 4MiB.
func newMetacacheWriter(out io.Writer, blockSize int) *metacacheWriter {
	if blockSize < 8<<10 {
		blockSize = 128 << 10
	}

		// Stat a volume entry.
		if err = Access(volumeDir); err != nil {
			return convertAccessError(err, errVolumeAccessDenied)
		}
	}

	// Use a small block size to start sending quickly
	w := newMetacacheWriter(wr, 16<<10)
	w.reuseBlocks = true // We are not sharing results, so reuse buffers.
	defer w.Close()
	out, err := w.stream()
	if err != nil {
		return err
	}
	defer xioutil.SafeClose(out)