Last week at VMworld and on the VMTN community I had a couple of questions around resource management and HA Admission Control. It appears people were using HA Admission Control for managing resources within their environment. In other words, the amount of VMs that HA would allow you to restart would be leading for managing resources. But is that what you should do?
If you look at how HA works and what HA is intended to do the answer in short is, No. Now the reason for this is that HA is all about getting your virtual machines up and running again. If you look at HA Admission Control in vSphere 5.0 you will quickly see that for instance the default value for CPU has been decreased from 256MHz to 32MHz, if no CPU reservations are specified that is. Now in many scenarios virtual machines will consume and demand more than that. Another thing to point out is that if no memory reservation is specified the memory overhead of the VM is used. These values are more than likely much lower than what your virtual machine currently consumes or demands. The thing to keep in mind is that these CPU and Memory values only represent what HA needs in order to power-on your virtual machines.
If you want to manage resources, avoid severe overcommitment, guarantee a certain experience you should start looking at the DRS statistics. You should start exploring tools like VC Ops, Cap IQ… Don’t (ab)use vSphere HA for this. It is not designed to solve this problem. One thing to think about though is maybe increasing the minimum value for slotsizes to avoid scenarios where environments are fully overloaded!? If you have a consolidation ratio in mind it should be fairly simple to figure out which value to use:
available memory esource per host / consolidation ratio = das.vmMemoryMinMB
available CPU esource per host / consolidation ratio = das.vmCpuMinMHz
I am not saying that you should do this, but I think it might not be a bad practice in environments where multiple people have access to vCenter and can deploy VMs. At least people will be triggered when you are running out of “slots” to start VMs.