VMware Cloud on AWS – TMCHAM – Part 6 – Sizing

In this edition of Things My Customers Have Asked Me (TMCHAM), I’m going to touch briefly on some things you might come across when sizing workloads for the VMware Cloud on AWS platform using the VMware Cloud on AWS Sizer.

VMware Cloud on AWS Sizer

One of the neat things about VMware Cloud on AWS is that you can jump on the publicly available sizing tool and input some numbers (or import RVTools or LiveOptics files) and it will spit out the number of nodes that you’ll (likely) need to support your workloads. Of course, if that’s all there was to it, you wouldn’t need folks like me to help you with sizing. That said, VMware has worked hard to ensure that the sizing part of your VMware Cloud on AWS planning is fairly straightforward. There are a few things to look out for though.

Why Do I See A Weird Number Of Cores In The Sizer?

If you put a workload into the sizer, you might see some odd core counts in the output. For example, the below screenshot shows 4x i3en nodes with 240 cores, but clearly it should be 192 cores (4x 48).

Yet when the same workload is changed to the i3 instance type, the correct amount of cores (5x 36 = 180) is displayed.

The reason for this is that the i3en instance types support Hyper-Threading, and the Sizer applies a weighting to calculations. This can be changed via the Global Settings in the Advanced section of the Sizer. If you’re not into HT, set it to 0%. If you’re a believer, set it to 100%. By default it’s set to 25%, hence the 240 cores number in the previous example (48 x 1.25 x 4 nodes).

Why Do I Need This Many Nodes?

You might need to satisfy Host Admission Control requirements. The current logic of Host Admission Control (as it’s applied in VMC sizer) is as follows:

  • A 2-host cluster should have 50.00 percent reserved CPU and memory capacity for HA Admission Control.
  • A 3-host cluster reserves 33.33 percent for HAC

And so on until you get to

  • A 16-host cluster reserving 6.25 percent of resources for HAC.

It’s also important to note that a 2-host cluster can accommodate a maximum of 35 VMs. Anything above that will need an extra host. And if you’re planning to run a full HCX configuration on two nodes, you should review this Knowledge Base article. Speaking of running things at capacity, I’ll go into Elastic DRS in another post, but by default we add another host to your cluster when you hit 80% storage capacity.

What About My Storage Consumption?

By default there are some storage policies applied to your vSAN configurations too. A standard Cluster with 5 hosts or less is set to 1 Failure / RAID-1, whilst a standard Cluster with 6 hosts or more is set to tolerate 2 Failures / RAID-6 by default. You can read more about that here.


There’s a bunch of stuff I haven’t covered here, including the choices you have to make between using RVTools and LiveOptics, and whether you should size with a high CPU to core ratio or keep it one to one like the old timers like. But hopefully this post has been of some use explaining some of the quirky things that pop up in the Sizer from time to time.