Q I'm trying to set up Unison so that it will synchronise backups on a small LAN. It all works fine, except that the transfer rate is very slow compared with the LAN's normal speed. The PCs are both using Unison on SUSE 9.2, connected by SSH. I'm getting about 80KB/sec on a 100MB/sec LAN that normally manages about 10MB/sec over NFS. Is the lack of speed due to SSH or to Unison? Is there any way to tweak whichever is causing the slowdown to improve matters?
A Depending upon the speed of the systems transfering data, the encryption overhead can cause the throughput rate to drop significantly, but for modern workstations and servers, it probably won't be noticable. However, when you're accessing the drive and possibly zipping up the contents, things can run slowly and start to drag. 80KB/sec is pretty dismal we doubt SSH has anything to do with this. You could try to run Unison over rsh or rsync and see if you're able to squeeze any more life out of it. Trying to copy some data using SSH directly will help to indicate if SSH is really any burden at all.