I am working on a project and we expect about 5 Million Users per day. It is a litte web application, written in php and we are currently using Apache to serve the requests.
My problem: I don't know how to calculate the hardware, I need to serve 5 Million Users a day. "Cloud" as in "go to Amazon" is not an option. But I thought about buying some big Servers and building our own Cloud with OpenStack or Eucalyptus.
How much Hardware do I need? Is virtualization an option?
Lastly: The webapp is mission critical - so I am going to double the hardware and do some failover - witch is a point for virtualization.