WARNING: gsutil rsync uses hashes when modification
time is not available at both the source and
destination. Your crcmod installation isn't using
the module's C extension, so checksumming will
run very slowly. If this is your first rsync since
updating gsutil, this rsync can take significantly
longer than usual. For help installing the
extension, please see "gsutil help crcmod".
I followed the instructions at https://cloud.google.com/storage/docs/gsutil/addlhelp/CRC32CandInstallingcrcmod hoping to get the fast C extension installed. But after following the Fedora instructions there, I still did not have the C extension for crcmod.
Troubleshooting and debugging eventually determined that the reason the C extension was not built and installed for crcmod was a missing file dependency, /usr/lib/rpm/redhat/redhat-hardened-cc1, which is provided by the redhat-rpm-config package.
So the solution to getting the fast crcmod C extension on Fedora 24 is to install redhat-rpm-config.
# dnf install redhat-rpm-config