I've seen backup servers maxed out by anywhere from 5 to 2500 hosts. There is no rule of thumb that would work everywhere or even for you without knowing more about your environment. What you do is you look for one of the following things to happen:
1. The I/O requirements of the backup indices/catalogs get too great for your server to handle. (If this is a Unix server, I'd tell you to run iostat and top. I would use the system performance monitor on NT/2000.)
2. You are backing up the maximum amount your bandwidth will allow. (100 Mb / 8 = 12 MB/s theoretical maximum. 12 MB X 60 seconds X 60 minutes X 8 hours = 345600. The maximum a 100 Mb connection could theoretically do is 345 GB in one night. If you're anywhere close to that, or even 70-80% of that, you've hit a wall.
3. The CPU or Memory on your box is completely gone, and you haven't experienced #2. If you can't upgrade the CPU or memory, it's time for another box. (By the way, #1 can cause #3.) Also, different backup software packages have different backup requirements.
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This was first published in July 2002