davecb's Journal: Capacity planning in six paragraphs

An acquaintance asked about what to measure, and what tools to use, expecting to hear about vmstar, sar or the like.

However, the really interesting measurements are of the application's performance: response time and transactions per second.

Imagine you have a web site which responds in 1/10 second on average, is known to be running on a single cpu (queuing center, to be precise) and is averaging 6 transactions per second (TPS)

From that you know that the maximum performance will be 10 TPS, because ten 1/10ths fit into one second. You also know you're at 60% of the maximum, a nice safe number.

Now correlate this with your average CPU usage, network bandwidth and IO bandwidth, and you have a little estimator for what resources are needed to maintain good performance.

You also know that things will start getting bad at >8 TPS, so if you expect more business in future, you need to add more queuing centers (CPUs) with the appropriate amounts of network and disk I/O bandwidth.

You can also now use both the resource usage figures and tools that all the other folks have suggested, and watch out for growth in each of them. If the trend in their use looks like it will soon get above the number that corresponds to 8 TPS, above, then and only then do you need to start buying resources.

--dave c-b