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Seeams that there is an issue in CPU hot-add on Windows Server 2016 running in VMware vSphere 6.5, but it’s something hard to reproduce this issues on a different systems.

Because on most systems it works correctly, but, at least in a case, the CPU hot add does not work as expected.

On the “faulty case” the behavior is similar to what is described on this old post on Reddit, but it was for Windows Server beta code. Or there was a similar problem reported also with vSphere 6.0 (see this post).

In my case was on a specific installation with:

  • HPE server, but the server it’s fine and in HCL
  • Windows Server 2016 (seems that the version does not case, same issue with updated or not updated version) in GUI mode
  • Windows installed in BIOS mode (have to try in EFI mode)
  • VMware vSphere 6.5U1 (not tried yet to upgrade it)
  • Latest VMware Tools (but seems not a problem with VMware Tools version)
  • Virtual hardware 13 – This seems the root cause!

I’ve tried to make some tests and seems that the issue appears when virtual hardware is version 13. On another environment, using v11 the CPU hot-add work correctly, but after upgrading to virtual hardware 13, the guest OS reboot each time you add a new vCPU, not necessary from one to two, but on each operation.

Could be interesting see if it’s only a bug of VMware vSphere or also something in Windows Server 2016…

Note that the VM console disconnect each time that you hot-add a vCPU, but this it’s almost expected.