Now that xxh hashes with their significant speed advantage are available and documented for rsync, I successfully performed (on FreeBSD12.1 amd64 and x86) # rsync -AXv --cc=xxh64 /tmp/file.txt /tmp/b2/ YAY! (Aside: for the reader, xxh64 is synonomous with xxhash) but # rsync -AXv --cc=xxh3 /tmp/file.txt /tmp/b2 unknown checksum name: xxh3 rsync error: requested action not supported (code 4) at checksum.c(86) [client=3.2.2] and # rsync -AXv --cc=xxh128 /tmp/file.txt /tmp/b2 performed similarly. As ~20% of devices are still 32bit (so would benefit from xxh3), based on the performance improvement suggested by https://cyan4973.github.io/xxHash/ we look forward to this enhancement. And as xxh64 functions, this PR should be a low priority. If it wasn't documented in the man page, I think rsync functions as intended ;)