- How much data are you backing up?
- How fast is your production storage to send the data to the backup server?
- How fast is your media server\proxy server to be able to process the data it is getting?
- How fast is your network to handle the flow of data (unless doing LAN-free backups)?
- How fast is your target storage for the backup files (doing disk-to-disk backups)?
- How fast is your network from the media server\proxy server to your target storage?
- How much data are you backing up each night?
- How often are you doing full backups?
- Are you doing active fulls or synthetic fulls?
- How big is your backup window?
- Is it ok to have your backups overflow into production hours?
- Do you need to backup using application agents?
- Are you backing up physical or virtual?
- If virtual are you using image-based backup solutions to get the VM?
- If virtual are you using VMware technologies like Change Block Tracking?
- It all comes down to "how fast can your infrastructure go?"
You can relate this question to one in your personal life when someone asks "How long is it going to take you to get there?" Well you can give a rough estimate, but there are many things to take into consideration, what is the rate of travel, what is the traffic like, which streets are you going to take, are you going to take a bike or your car or a bus or are you going to walk? All of these are similar to your backups. It depends on the mode at which your going to get your data from point A to point B and how congested is that route?
Now it gets much easier when these questions are answered and the more information you give us the more detailed response you'll get. But nothing beats good 'ol testing. Test your environment. See how long it takes to back up one server, ok see how long it takes to do that incremental, ok now see how long it takes to backup two servers at the same time. When running things in parallel in the IT world it is not a linear improvement. Running one backup might take 30 minutes, running two backups might take 35 minutes and running three backups might take 45 minutes. There will definitely be a point of diminishing returns and that is where testing will prove to have paid off. You will get to a point where it takes you longer to back up than if you backed off one or two servers and waited for the one of them to finish running. Where that point is is entirely up to your hardware, most likely it is not the software. Most backup software these days are built for multi-threading and to utilize the resources as efficiently as possible but you can't magically squeeze more out of your hardware that what its capacity is. Now some products offer compression and deduplication that will allow you to technically send more data than others.
I hope you enjoyed my little lesson here. It is something I learned when I came to the dark side (more on this later).