When you back up network data, you have two basic approaches to running the backup software: You can perform a local backup, in which the backup software runs on the file server itself and backs up data to a tape drive that’s installed in the server.
Or, you can perform a network backup, in which you use one network computer to back up data from another network computer. In a network backup, the data has to travel over the network to get to the computer that’s running the backup.
If you run the backups from the file server, you’ll tie up the server while the backup is running and users will complain that their server access has slowed to a snail’s pace.
On the other hand, if you run the backup over the network from a client computer or a dedicated backup server, you’ll flood the network with gigabytes of data being backed up. Your users will then complain that the entire network has slowed to a snail’s pace.
Network performance is one of the main reasons why you should try to run your backups during off hours, when other users aren’t accessing the network. Another reason to run backups during off hours is so that you can perform a more thorough backup. If you run your backup while other users are accessing files, the backup program is likely to skip over any files that are being accessed by users at the time the backup runs.
As a result, your backup won’t include those files. Ironically, the files most likely to get left out of the backup are often the files that need backing up the most because they’re the files that are being used and modified.
Here are some extra thoughts on client and server backups:
Backing up directly from the server isn’t necessarily more efficient than backing up from a client because data doesn’t have to travel over the network. The network may well be faster than the tape drive. The network probably won’t slow down backups unless you back up during the busiest time of the day, when hordes of network users are storming the network gates.
To improve network backup speed and to minimize the effect that network backups have on the rest of the network, consider using a 1,000 Mbps switch instead of a normal 100 Mbps switch to connect the servers and the backup client. That way, network traffic between the server and the backup client won’t bog down the rest of the network.
Any files that are open while the backups are running won’t get backed up. That’s usually not a problem because backups are run at off hours when people have gone home. One way to solve this problem is to set up the server so that it automatically logs everyone off the network before the backups begin.
Some backup programs have special features that enable them to back up open files. For example, the Windows Server 2003 and 2008 backup programs do this by creating a snapshot of the volume when it begins, thus making temporary copies of any files that are modified during the backup. The backup backs up the temporary copies rather than the versions being modified. When the backup finishes, the temporary copies are deleted.
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Source:http://www.dummies.com/how-to/content/network-data-local-versus-network-backups.html
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