capacity planning
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Appendix B
Capacity Planning Calculations
In Chapter 11, Determining Headroom for Applications, we covered how to deter-
mine the headroom or free capacity that was available for your application. In this
appendix, we will walk through a larger example of capacity planning for an entire
site, but we will follow the process outlined in Chapter 11. The steps to be followed are
1. Identify components
2. Determine actual and maximum usage rates
3. Determine growth rate
4. Determine seasonality
5. Compute amount of headroom gained through projects
6. State ideal usage percentage
7. Perform calculations
For our example, let’s use our made-up company AllScale.com, which provides
Software as a Service (SaaS) for human resources professionals. The site is becoming
very popular and growing rapidly. The growth is seen in bursts; as new companies
sign up for the service, the load increases based on the number of human resource
managers at the client company. So far, there are 25 client companies with a total of
1,500 human resource managers that have accounts on AllScale.com. The CTO
needs to perform a capacity planning exercise because she is planning for next year’s
budget and wants accurate cost projects.
Step 1 is to identify the components within the application that we care about suf-
ficiently to include in the analysis. The AllScale.com application is very straightfor-
ward with a Web server tier, application server tier, and single database with standbys
for failover. AllScale.com was migrated this past year to a new network and the net-
work devices, including the load balancers, routers, and firewalls, were all purchased
to scale to 6x current maximum traffic. We will skip the network devices in this
capacity planning exercise, but periodically they should be reanalyzed to ensure that
they have enough headroom to continue to scale for AllScale.com’s growth.
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