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Is there a rule of thumb for planning for server capacity? For example, if I know that I need five "beefy" servers to handle my in-house needs, should I buy an extra one or two -- or 20% more CPU/memory than needed -- for server redundancy? Assume that we don't expect to grow in terms of employees or clients, or that we have already handled that. Also, assume that we have all the spare parts we need -- this would just be to have on hand for additional capacity.
This depends entirely on your situation, as there is no all-encompassing rule of thumb when it comes to capacity planning. Also, you never mentioned how much if any of your server infrastructure will be virtualized. That's a huge variable when attempting to plan for server capacity.
Given the assumptions you've outlined, I would focus the entire capacity planning effort on the concept of server redundancy. This means a determination must be made with regard to what portions of your organization's infrastructure are vital and what portions are simply nice to have. All things being equal, I would allocate 25% more CPU resources than what your organization currently needs toward your most vital infrastructure. Then I would examine what has been deemed a "nice to have," and I would allocate approximately 15% of extra CPU resources toward this portion of your organization.
Another item you have to think about with regard to redundancy is storage. Again, all things being equal, I would calculate the total amount of hard drive space within your network and allocate some sort of network resource(s) with an amount of storage space equal to or greater than the total you previously calculated.
About the author:
Brad Casey is an expert on network security with experience in penetration testing, public key infrastructure, VoIP and network packet analysis. He also covers system administration, Active Directory and Windows Server 2008, with interest in Linux virtualization and Wireshark captures. He spent five years in security assessment testing for the U.S. Air Force.
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