code: func (file *GridFile) SetChunkSize(bytes int) {
  	file.assertMode(gfsWriting)
  	debugf("GridFile %p: setting chunk size to %d", file, bytes)
  	file.m.Lock()
  	file.doc.ChunkSize = bytes
  	file.m.Unlock()
  }
docs: // SetChunkSize sets size of saved chunks.  Once the file is written to, it
  // will be split in blocks of that size and each block saved into an
  // independent chunk document.  The default chunk size is 256kb.
  //
  // It is a runtime error to call this function once the file has started
  // being written to.