Fastest backup for large files

I consider rsync to be one of the best options in this case. You may want to skip the Mac OS X built in rsync 2.6.9 and go for rsync 3 which is usually way faster. You can get a precompiled Mac optimized universal binary of rsync 3.0.6 with mlbackup, a free rsync based backup utility. (Installs rsync in /usr/local/maclemon/bin/rsync) For full and fair disclosure: I am the author of mlbackup.

If this server is your live mailserver, you may want to do an rsync copy to your FireWire device and jut let it take the time it needs to (and take time it will). When you're ready to go you just rerun the rsync command to have it updated to the current status, which will take significantly less time.

I'd suggest you just rsnyc to another drive, and run the mailserver from the new drive. That way you only need to copy once. Don't know if you have a spare bay left in your Xserve, or if you can run from the external drive for a few days. With this technique you can slit the backup and restore-to-internal-drive processes into to distinct events which you could probably do on two weekends to keep service downtime at a minimum.

Just make sure you have more than one backup of the data, just in case. You don't want to have anything go wrong.

You can also have a look at Alex's excellent mailbfr which will work up until 10.5.8, a Snow Leopard compatible version is in the works and should be released in the not too distant future.

You can't "gzip the whole thing" as gzip only compress one file, you could create a tar file and gzip it to "gzip the whole thing" but you would loose rsync capability of copying only modified file.

So the question is: is it better to store file I need to rsync gziped or rely on -z option of rsync.
The answer is probably that you don't want the file unzipped on your server ? I guess yes, so I don't see how you could manage to gzip file before doing the rsync.

May be you don't need the rsync capability of copying only modified file ? In this case why using rsync instead of doing a scp of a tar.gz file containing your stuff ?

Anyway to answer the question, rsync gzip will be a little less efficient than gziping file with gzip. Why ? because rsync will gzip data chunk by chunk, so a smaller set of data will be used to create the table that gzip use to do compression, a bigger set of data (gzip would use the whole file at once) give a better compression table. But the difference will be very very small in most case but in very rare case the difference can be more important (if you have a very large file with very long partern repeating many time on the file but far away from each other) (This is a very simplified example)