Rsync Performance

[Joe Eckstrom]

I'm looking at using Rsync to synchronize 600 Windows servers across the country to a central location.  I'll be synchronizing about 15-20 gigs of data, but the vast majority will never change... every now and then, we would have to add new files, ranging in size from a few bytes to 1.5GB.  Each server is on a slow frame relay connection and will be limited to 3KB/sec.  The connections are reasonably reliable, however we would definitely want the ability to resume an incomplete download.  The central rsync server could run Linux if there was a performance benefit.

What kind of a load could a single fairly beefy server take?  It'd have dual 3+ghz processors, a couple gigs of memory, striped and mirrored SCSI RAID.  I'm estimating that at peak times we'll range between 20-25Mbps, possibly as much as 35.  I could probably divide the load between a few servers, but I'd like to stick with one if it would work.  Does rsync running as a daemon take advantage of multiple processors?

What kind of overhead does compression add to a session?  Does wrapping rsync in ssh cause as much of a performance hit as compression?  

What's the difference between --bwlimit and --bwlimit-mod?

Thanks a lot for your help!
Joe Eckstrom

DaVita Inc.