Last week I received three different questions about vSphere HA Admission control and I figured I would lay out the basics once more. What is admission control?
vCenter Server uses admission control to ensure that sufficient resources are available in a cluster to provide failover protection and to ensure that virtual machine resource reservations are respected.
Almost every thing you need to know about admission control is in that single sentence. But lets break it down in to more consumable bites:
- vCenter uses admission control to ensure that sufficient resources are available in a cluster to provide failover protection.
- vCenter uses admission control to ensure that virtual machine resource reservations are respected.
So first and foremost… Admission control is not about resource management, I devoted a whole article to that so not going in to details, but HA admission control is all about reserving resources to allow for a failover.
Secondly, admission control ensures virtual machine resource reservations of powered-on VMs can be respected. This is because virtual machine resources reservations are required to be available in order for a power-on to successfully complete! Meaning that if you set a 5GB memory reservation there needs to be 5GB of unreserved memory available (+ reserved memory overhead) on a single host in order for this virtual machine to power-on. If that 5GB machine is actually actively using 40GB it might end up swapping / paging, as only those 5GB of reserved capacity is taken in to account!
Note the “+ reserved memory overhead”! Every virtual machine has a memory overhead. This is usually in the range of a couple hundred MBs. For a successful power-on attempt you will need to be able to reserve this memory. If there is not enough “unreserved memory capacity” the power-on attempt will fail. So in reality that 5GB could just be 5.15GB. Might seem irrelevant, but I will explain why it is relevant in a second. Did you spot the “powered-on”? Yes, admission control only takes the resource reservations of powered-on VMs in to account. So if you have a VM with a large memory reservation which is powered-off it will not impact your admission control calculations!
- Admission Control is about reserving resources to allow for a fail-over.
- Admission Control is no resource management tool, it only takes reserved capacity of powered-on VMs in to account.
So now that you know what admission control is. There are three policies when it comes to admission control… and we will discuss these in Part 2 of this article.