I've got a folder with a very large number of files and subdirectories (it's full of maildir folders for my company actually) which has files that rank in the millions. I need to replicate this to another server as a backup.

The number of files is not the only consideration, the volume is quite significant, even over gigabit.

I've tried using rsync to try to avoid re-copying a large volume of data by only taking the differences across to the backup server, but the extremely large number of files tends to hurt rsync and take forever while it's building up file lists.

Does anybody have a better idea or another tool for replicating a directory structure with both large volume and a large number of files?

I think that the quickest solution would be to make a tar backup of the directory onto a fast medium such as a SATA or PATA hard drive. That would free up the active directory as quickly as possible. Then copy this tar file to a removable medium such as a USB external drive and carry it to the backup server. Lastly, of course, you would restore the tar archive onto the backup server.

The advantages of this are that the network will not be flooded with this maintenance operation which will allow users to have full speed network access to the active server, this would be the quickest approach which would mean that the active server bogged down with this maintenance operation for the least amount of time, and you end up with a tar archive of the mail directory which you can store.

I recommend using the nice utility to start operations such as copying the archive file to removable medium. Otherwise the server will be too busy performing this operation to provide good service to the network users.

Solution two is to add a second NIC to the server and put that on a physically separate LAN to a second NIC on the backup server. This could be a point to point network and it could use gigabit speed network components. This would be pretty fast which would result in minimal time spend performing the backup or copy from the active server to the backup server. I worked one place that did this for their servers. It worked great! The backup network wasn't point to point, it was a normal LAN with a router but it was fast and it didn't interfere with the building LAN.