When to add more device hosts
Again using the NetBackup illustration, the good news is that adding device hosts or media servers is not as daunting a task as determining whether or not to divide your backup domain. Typically when you find that you aren’t meeting your backup window, but you have plenty of capacity in your tape library, it’s a sure sign that you need to add a media server. By performing some very basic mathematical formulas, you can determine the approximate amount of data your current environment can actually handle. First, identify any bottlenecks in your backup environment before you decide that you need to invest in more of anything.
A single 100-Mbps interface is capable of handling approximately 12.5 MB/second, or roughly two DLT8000 drives at native speeds of 6 MB/ second. This definitely looks like a potential bottleneck, and one would more than likely benefit by having an additional media server in place to share the burden.
With a GbE between the backup server and the switch you can drive 125 MB/sec into the backup server, or approximately 20 DLT 8000 tape drives. Naturally, all of this would be dependent on the I/O interface to the tape drives, but assuming you had multiple SCSI interfaces or were using Fiber-connected drives, the numbers would hold fairly true. Would you really want to drive 20 DLT drives from one backup server? Probably not. At that point you would want to expand to another media server or another tape drive type.
Either by inheritance, slow growth, or, quite frankly, a very tight budget year, we will one day be faced with a backup environment that hops across networks. When the time comes, and budgets are loosening up, you definitely want to consider introducing a media server into this type of environment. Not only are you backing up another network, but also you are pulling all of that data across the network back to the media server to be written to tape. Wouldn’t it be better to have that data remain within the network of its origin and only have to send the meta data back to the master server? We hope you answered yes. If you didn’t, you are probably still under very tight budgetary constraints