Johnny Zhang
In vCenter 4.0 we now have some easy to used monitor tools for ESX server performance. One of the chart is the memory usage
This usages is NOT the active memory that is currently used by the ESX server or the VM. So when you see very high usage, don't be panic.
In fact, this chart shows the total "Consumed Host Memory" by all the VMs running on the host.
As we can see from the graph the "Consumed Host memory" is 2277.00MB and the "Active Guest Memory" is only 122.00MB. Why a big different?

This is by design, the "Consumed Host Memory" shows the highest memory used by that VM.
OS such like Windows will touch all the memory assigned to it during start up, so most likely you will see the consumed memory is close or little more then the memory assigned to the VM (Overhead to power on the VM is also added here). ESX server will NOT take back the memory if there is no resource contention on the server. The reason is, once the VM need more memory it can access them faster than send the request to vmkernel again.
If you want to know the real time memory usage you can look into the advanced chart to find it out.
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