[lug] rsync throughput curiosity

[Lee Woodworth]

Something I hadn't noticed before is that rsync throughput can be cpu
limited when using ssh.

I just ran into this copying planet*.osm.pbf on the internal
network to the bulk storage server (which can sustain 125Mbyte/s
writes for several seconds).

The copies from the source to a  much faster development system run
at ~700mbit/s (possibly limited by cat5e cabling, mtu is 9000 on the
source and dest). A copy to the storage server via an nfs mount point
runs at ~610mbit/s. But a copy using rsync  directly to the storage
server with the default cipher runs at ~200mbit/s.

The storage server is a low-wattage AMD GX-412TC SOC @ 1ghz w/4gb
and aes_ni instruction support. It's not clear if openssl is using the
aes instruction.

One core gets pegged at 100% when using the default cipher:
     rsync --archive --stats  --rsh="ssh -c 'chacha20-poly1305 at openssh.com'"
The transfer rate from the source runs ~200mbit/s

Rebuilding openssl with gmp support and using a different cipher:
     rsync --archive --stats  --rsh="ssh -c 'aes256-gcm at openssh.com'"
The transfer rate is now ~500mbit/s

Using aes128-gcm at openssh.com results in a ~533mbit/s rate.