@Override
    public void close() {
      try {
        ByteBufferUtil.free(buffer);
      } catch (IOException e) {
        LOGGER.info("Unable to unmap chunk", e);
      }
      try {
        channel.close();
      } catch (IOException e) {
        LOGGER.warn("Error closing chunk", e);
      }
    }
Memory-map the chunk file, then iterate through all features in it.
planetiler-core/src/main/java/com/onthegomap/planetiler/collection/ExternalMergeSort.java