Yellow Bricks

by Duncan Epping

admission control

HA Admission Control the basics – Part 1/2

· ·

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:

  1. vCenter uses admission control to ensure that sufficient resources are available in a cluster to provide failover protection.
  2. 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!

In summary:

  1. Admission Control is about reserving resources to allow for a fail-over.
  2. 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.

I selected “failover host” and my VMs still end up on a different host after an HA event

· ·

I received a question today about HA admission control policies, and more specifically about the “failover host” admission control policy. The question was why VMs were restarted on a different host then selected with the “Failover Host” admission control policy. Shouldn’t this policy guarantee that a VM is restarted on the designated host?

The answer is fairly straight forward, and I thought I blogged about this already but I cannot find it so here it goes. Yes, in a normal condition HA will request the designated failover host to restart the failed VMs. However there are a couple of cases where HA will not restart a VM on the designated failover host(s):

  • When the failover host is not compatible with the virtual machine (portgroup or datastore missing)
  • When the failover host does not have sufficient resource available for the restart
  • When the virtual machine restart fails HA retries on a different host

Keep that in mind when using this admission control policy, it is no hard guarantee that the designated failover host will restart all failed VMs.

Host Admission Control: Powering on a VM…

· ·

I was reading a whitepaper by VKernel and it mentioned the following “a failover host for these VMs requires sufficient idle resources“. In this whitepaper it is discussed how Monster VMs pose challenges for both HA and DRS. As I had a similar question last week at VMworld I figured I would post this. Also because it is fundamental to understand this with regards to HA. Now the thing is, I agree that there is no point in creating large VMs just because you can. Without a doubt do Monster VMs pose challenges with regards to managing resources. However I do want to point out that technically speaking the statement is incorrect.

To power-on a VM you need unreserved memory capacity! The unreserved memory capacity needs to be equal to the memory reservation of the VM and the memory overhead! In other words, if you set no memory reservation you can power-on multiple 96GB VMs on a 48GB host. Just because the memory overhead is much lower than 48GB of memory. Now this doesn’t mean it is a best practice, or this is something I would recommend, but it does mean that if you look at how HA handles a fail-overs it will accommodate the restart of these virtual machine. This also means that with regards to HA Admission Control, chances of not being able to power-on your virtual machine because of insufficient resources are fairly slim. I bet that if you over-commit to such an extent that a power-on operation is impossible you have a lot more challenges to begin with!

Frank Denneman wrote a nice article about this a while back, it explains perfectly what the impact is of a memory reservation.