Application end-users generally have some performance expectations. Often you can numerically quantify them. To ensure that customer needs are met, you must understand these expectations clearly, and use them in capacity planning.
Consider the following questions regarding performance expectations:
What do users expect the average response times to be for various interactions with the application? What are the most frequent interactions? Are there any extremely time-critical interactions? What is the length of each transaction, including think time? In many cases, you may need to perform empirical user studies to get good estimates.
What are the anticipated steady-state and peak user loads? Are there are any particular times of the day, week, or year when you observe or expect to observe load peaks? While there may be several million registered customers for an online business, at any one time only a fraction of them are logged in and performing business transactions. A common mistake during capacity planning is to use the total size of customer population as the basis and not the average and peak numbers for concurrent users. The number of concurrent users also may exhibit patterns over time.
What is the average and peak amount of data transferred per request? This value is also application-specific. Good estimates for content size, combined with other usage patterns, will help you anticipate network capacity needs.
What is the expected growth in user load over the next year? Planning ahead for the future will help avoid crisis situations and system downtimes for upgrades.