private void loadNextSegmentV1() throws IOException {
    // just read raw data as input
    if (channel.position() >= fileSize) {
      throw new IOException("Unexpected end of file");
    }
    if (Objects.isNull(dataBuffer)) {
      // read 128 KB
      dataBuffer = ByteBuffer.allocate(128 * 1024);
    }
    dataBuffer.clear();
    readWALBufferFromChannel(dataBuffer);
    dataBuffer.flip();
  }
1 byte for whether enable compression, 4 byte for compressedSize
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/storageengine/dataregion/wal/io/WALInputStream.java