Performance versus capacity
Now that we know what IaaS performance is, what metric should we use to measure it?
A lot of customers mistake capacity with performance. They associate low utilization with high performance, for example:
- Low ESXi CPU utilization means its performance is good (it is fast)
- Low VM CPU utilization means its performance is good (it is fast)
If you ponder these points, you will see the failure in the logic. There are several reasons why your ESXi utilization is irrelevant:
- Whether the ESXi has high or low utilization has nothing to do with its performance. An ESXi does not become slower as its utilization goes from 5 percent to 50 percent. It's still running at the same speed!
- An ESXi with low utilization has a better chance of serving all its VM better than an ESXi with high utilization. There is certainly a correlation. The question is, how do you quantify that correlation? You cannot say that 25 percent ESXi CPU utilization means none of its VMs has CPU contention...
