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()
}
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()
}
// 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.