Answering some admission control questions

I received a bunch of questions on HA admission control in this blog post and I figured I would answer them in a blog post so that everyone would be able to find / read it. This was the original set of questions:

There are 4 ESXi Hosts in the network and 4 VMs (Same CPU, RAM Reservation for all VMs) on each Host. Admission Control is policy is set to ‘Host failure cluster tolerates’ to 1. All the available 12 slots have been used by the powered ON VMs, except the 4 reserved slots for failover.
1) What happens if 2 ESXi Hosts fails now? ( 2 * 4 VMs needs to fail over). Will HA restart only 4 VMs as it has only 4 slots available? And Restart of the remaining 4 VM fails?
Same Scenario, but Policy is set to ‘% of cluster resources reserved’ = 25%. All the available 75 % resources have been utilized by all the 16 VMs, except 25 % reserved for failover
2) What happens if 2 ESXi Hosts fails now? ( 2 * 4 VMs needs to fail over). Will HA restart only 4 VMs as it consumes 25 % of resources? And Restart of the other 4 VM fails?
3) Does HA check the VM reservation (or any other factor) at the time of restart ?
4) HA only restart a VM if the Host could guarantee the reserved resources or restart Fails?
5) What if no VM reservations are set VM level ?
6)What does HA takes into consideration when it has to restart VMs which has no reservation ?
7)Will it guarantee the configured Resources for each VMs ?
8)If not, How HA can restart 8 VMs (as per our eg) when it only has configured reserved resources for just 4 VM
9)Will it share the reserved resources across 8 VMs and will not care about the resource crunch or is it about first come first serve
10)Admission control doesn’t have any role at all in the event of HA failover ?

Let me tackle these questions one by one:

1. In this scenario 4 VMs will be restarted and 4 VMs might be restarted! Note that the “slot size” policy is used and that this is based on the worst case scenario. So if your slot is 1GB and 2GHz but your VMs require way less than that to power-on it could be all VMs are restarted. However, HA guarantees the restart of 4 VMs. Keep in mind that this scenario doesn’t happen too often, as you would be overcommitting to the extreme here. As said HA will restart all VMs it can. It just needs to be able to satisfy the resource reservations on memory and CPU!
2. Again, also in this HA will do its best to restart. It can restart new VMs until all “unreserved capacity” is used. As HA only needs to guarantee reserved resources chances of hitting this is very slim, as most people don’t use reservations at a VM level it would mean you are overcommiting extremely
3. Yes it will validate if there is a host which can back the the resource reservations before it tries the restart
4. Yes it will only restart the VM when this can be guaranteed. If it cannot be then HA can call,”DRS” to defragment resources for this VM
5. If there are no reservations then HA will only look at the “memory overhead” in order to place this VM
6. HA ensures the portgroup and datastore are available on the host.
7. It will not guarantee configured resources, HA is about restarting virtual machines not about resource management. DRS is about resource management and guaranteeing access to resources.
8. HA will only be able to restart the VM if there are unreserved resources available to satisfy the VMs request
9. All resources required for a virtual machine need to be available on a single host! Yes resources will be shared on a single host, just as long as no reservations are defined.
10. No Admission Control doesn’t have any role in an HA failover. Admission Control happens on a vCenter level, HA failovers happen on an ESX(i) level.