See [SO](https://www.google.com/url?q=http://stackoverflow.com/questions/39578154/googles-gsutil-cp-command-runs-a-checksum-on-the-file-transferred-does-googl/39578458%2339578458&sa=D&usg=AFQjCNGv6SaRIIFtX9rCAJ7dFNb1ZHyx-w) for context.
See SO for context.