Capacity planning needs to provide answers to two questions: What are you going to need to buy in the coming year? And when are you going to need to buy it?
To answer those questions, you need to know the following information:
Current usage: Which components can influence service capacity? How much of each do you use at the moment
Normal growth: What is the expected growth rate of the service, without the influence of any specific business or marketing events? Sometimes this is called organic growth.
Planned growth: Which business or marketing events are planned, when will they occur, and what is the anticipated growth due to each of these events?
Headroom: Which kind of short-term usage spikes does your service encounter? Are there any particular events in the coming year, such as the Olympics or an election, that are expected to cause a usage spike? How much spare capacity do you need to handle these spikes gracefully? Headroom is usually specified as a percentage of current capacity.
Timetable: For each component, what is the lead time from ordering to delivery, and from delivery until it is in service? Are there specific constraints for bringing new capacity into service, such as change windows?
From that information, you can calculate the amount of capacity you expect to need for each resource by the end of the following year with a simple formula:
Future Resources = Current Usage x (1 + Normal Growth + Planned Growth) + Headroom
You can then calculate for each resource the additional capacity that you need to purchase:
Additional Resources = Future Resources ñ Current Resources
Perform this calculation for each resource, whether or not you think you will need more capacity. It is okay to reach the conclusion that you don't need any more network bandwidth in the coming year. It is not okay to be taken by surprise and run out of network bandwidth because you didn't consider it in your capacity planning. For shared resources, the data from many teams will need to be combined to determine whether more capacity is needed.
Before you can consider buying additional equipment, you need to understand what you currently have available and how much of it you are using. Before you can assess what you have, you need a complete list of all the things that are required to provide the service. If you forget something, it won't be included in your capacity planning, and you may run out of that one thing later, and as a result be unable to grow the service as quickly as you need.
What to track
If you are providing Internet based services, the two most obvious things needed are some machines to provide the service and a connection to the Internet. Some machines may be generic machines that are later customized to perform given tasks, whereas others may be specialized appliances.
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