Performance testing and identifying the cause of the brakes?

Question

System administration

The server is running under VMware ESX, which is not available to me.
On the server: UbuntuServer 10.04 + nginx + redmine + svnserve.
The last two weeks everything has become terribly slow, how can you identify the problem?
I monitor using Cacti, but I noticed that the graphs do not really correspond to reality (100% load on the server on the graphs can be either 10% or 20% and 60%).
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Answer 1

Probably on the ESX host, the memory goes into swap. Inside the virtual machine, this may not be seen, there the memory can be represented as physical. Chat with the ESX admin.

Answer 2

It cannot be that on the charts there are 10 percent of the load, but in reality 100, check the materiel.