Wednesday, March 17, 2021

Overprovisioning vCPUs in ESXi as a VMWare guest


I have a large server (96 vcpus and 1TB RAM) for working on cloud projects. A limitation, however, is that VMWare Workstation Pro 16 has a limitation of allocating a max of 32 vCPUs and 128GB RAM to any one guest VM. Normally this isn't a problem, but when you're dealing with OpenShift and Watson AIOps on-prem install, that's not enough. Specifically, Watson AIOps states that it requires a minimum of 36 cores for the all of the VMs required.


It turns out that the 32 vCPU limit isn't really a problem in this case. VMWare products allow you to overprovision the host resources to guests. And ESXi doesn't have any limitations on the number of vCPUs or amount of memory you can assign to one of its guests. This means that I can run ESXi as a guest under VMWare Workstation, and allocate more than the 32 vCPUs to its guest VMs. Here's a picture that I think clears up the situation:

As you can see, my ESXi guest (32 vCPUs) has three guest VMs that are using a total of 42 vCPUs, and they're all running fine. If all of the vCPUs get busy, performance will degrade, but I don't expect that to ever happen in my lab.

I've seen discussions where people overprovision vCPUs in a ratio of up to 4:1, meaning that I could possibly allocate 128 vCPUs in my ESXi guest as long as the guest VMs aren't too busy.

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