Death to false myths: Admission Control lowers consolidation ratio

Death to false myths probably sounds a bit euuhm well Dutch probably, or “direct” as others would label it. Lately I have seen some statements floating around which are either false or misused. One of them is around Admission Control and how it impacts consolidation ratio even if you are not using reservations. I have had multiple questions around this in the last couple of weeks and noticed this thread on VMTN.

The thread referred to is all about which Admission Control policy to use, as the selected policy potentially impacts the amount of virtual machines you can run on a cluster. Now lets take a look at the example in this VMTN thread, and I have rounded up some of the numbers to simplify things:

• 7 host cluster
• 512 GB of memory
• 132 GHz of CPU resources
• 217 MB of Memory Overhead (no reservations used)

So if you do the quick math. According to Admission Control (host failures example) you can power-on about ~2500 virtual machines. That is without taking N-1 resiliency in to account. When I take out the largest host we are still talking about ~1800 virtual machines that can be powered on. Yes that is 700 slots/virtual machines less due to the N-1, admission control needs to be able to guarantee that even if the largest host fails all virtual machines can be restarted.

Considering we have 512GB in total that means that if those 1800 virtual machines on average actively use 280MB we will see TPS / swapping / ballooning / compression. (512GB / 1800 VMs) Clearly you want to avoid most of these, swapping / ballooning / compression that is. Especially considering most VMs are typically provisioned with 2GB of memory or more.

So what does that mean or did we learn? Two things:

• Admission Control is about guaranteeing virtual machine restarts
• If you set no reservation you can power-on an insane amount of virtual machines

Let me reemphasize the last bullet, you can power-on an INSANE amount of virtual machines on just a couple of hosts when no reservations are used. In this case HA would allow for 1800 virtual machines to be powered-on before it starts screaming it is out of resources. Is that going to work in real life, would your virtual machines be happy with the amount of resources they are getting? I don’t think so… I don’t believe that 280MB of physically backed memory is sufficient for most workloads. Yes, maybe TPS can help a bit, but chances of hitting the swap file are substantial.

Let it be clear, admission control is no resource management solution. It is only guaranteeing virtual machines can be restarted and if you have no reservations set then the numbers you will see are probably not realistic. At least not from a user experience perspective. I bet your users / customers would like to have a bit more resources available than just the bare minimum required to power-on a virtual machine! So don’t let these numbers fool you.