Capacity planning is an important part of running a network. To me, it’s all about two things: Fewer surprises, and better business discussions. If you can do that, you'll get a lot more respect.


When I was working for an ISP, we had several challenges:

  • Average user Internet usage is steadily increasing
  • Users are moving to higher-bandwidth access circuits, which means even more usage
  • Upstream WAN bandwidth still costs money. Sometimes lots of money.

I built up a capacity planning model that took into account current & projected usage, and added in marketing estimates of future user changes. It wasn’t a perfect model, but it was useful. It gave me something to use to figure out how we were tracking, and where the pain points would be.


Fewer Surprises

No-one likes surprises when running a network. If your VM runs out of memory, it’s easy enough to allocate more. But if your WAN link reaches 90%, it might take weeks to get more bandwidth from your provider. If you hit that peak due to foreseeable growth, it makes for an awfully uncomfortable discussion with the boss. Those links can be expensive too. You'll be in trouble with the bean-counters if the budgets have been set, and then you tell them that you need another $10,000/month. You can't always get it right. There’s always situations where a service proves far more popular than expected, or a marketing campaign takes off. But reducing the surprises helps your sanity, and it improves your credibility.


Better Business Discussions

I like to use capacity planning and modeling tools for answering those “What if?” questions. e.g. The marketing team will come to you with questions like these:

  • What if we add another 5,000 users to this service? What will that do to our costs?
  • What if we move 10,000 customers from this legacy service to this new one? How will our traffic patterns change?
  • Do we have capacity to run a campaign in this area? Or should we target some other part of the country?
  • Where should we invest to improve user experience?


If you've been doing your capacity planning, then you've got the data to help answer those questions. You get a lot more respect when you're able to have those sorts of discussions, and answer questions sensibly.


This does take real effort though. Getting the data together and making sense of it can be tough. Tying it to business changes in particular is tough. No capacity planning model fully captures everything. But it doesn't have to be perfect - you can always refine it over time.


Are you actively doing capacity planning? How is it helping? (Assuming it is!) If you're not doing any capacity planning, what’s been holding you back? Or have you had any really nasty surprises, where you've run out of capacity in an embarrassing way?