SSH cipher speed

When setting up backups over SSH (e.g. rsnapshot with rsync over SSH), it's important to know that the default SSH cipher isn't necessarily the fastest one. In this case, the CPU-based encryption is the performance bottleneck, and making it faster means getting faster backups.
A test (copying a 440 MB file between a fast Xeon CPU (fast=no bottleneck there) and an Atom based NAS) shows that the arcfour family of ciphers are clearly the fastest in this setup:

cipher	real time	user time	bandwidth
arcfour	0m9.639s	0m7.423s	45.7 MB/s
arcfour128	0m9.751s	0m7.483s	45.1 MB/s
arcfour256	0m9.856s	0m7.764s	44.7 MB/s
blowfish-cbc	0m13.093s	0m10.909s	33.6 MB/s
aes128-cbc	0m22.565s	0m20.129s	19.5 MB/s
aes128-ctr	0m25.400s	0m22.951s	17.3 MB/s
aes192-ctr	0m28.047s	0m25.771s	15.7 MB/s
3des-cbc	0m51.067s	0m48.018s	8.6 MB/s

The default configuration of openssh uses aes128-ctr, so changing the cipher to arcfour gets me a 2.5-fold increase in bandwidth here ! Use the "Ciphers" keyword in .ssh/config or the "-c" command line parameter to change the order of preference of the available ciphers. YMMV.

As a reference (cfr. deinoscloud's comment), I ran "nc -l -p 3333" on the Atom side, and ran "cat file | nc atom 3333" on the Xeon:

cipher	real time	user time	bandwidth
cleartext	0m4.135s	0m0.311s	106.5 MB/s

. This shows that in the cleartext case, the CPU (user) time is not the bottleneck, and we're very close to using the full 1Gbps bandwidth.