We have an active/active failover cluster and plan to add more databases on these clusters but the current hardware resource will possibly need to be upgraded. How can we provide performance metrics to upgrade the hardware subsystem if the current resource is okay but adding additional databases will be a problem? How do I justify this? Please help. Thank you.
If you don't already have a baseline on performance for the current systems, that's the place to start. I'd focus on memory and disk, especially understanding how tempdb works. CPU, on most systems, is never really stressed, but memory and disk almost always will be. If you're planning on adding other systems, are those systems already in production elsewhere? Then measure them as well, again, focusing on disk and memory with a special emphasis on tempdb. Or, are they under development? Then, measure them in your dev or QA area (I usually prefer getting those measures in QA). You should be able to do this from a position of knowledge and not have to make any guesses at all.
You need to make a "best
estimate". Why do you think it will be necessary? You must have some experience or evidence that this might be required so document that as soon as you can and work from there. You can, using a test environment, have the current workload and measure/benchmark the hardware performance and then add the new systems and measure/benchmark again. You can then apply the delta changes to the live system and suggest you might see a similar change in its performance. Which part of the hardware are you most concerned about?