Adding vCPUs to an existing OS install

Solution 1:

Merely adding or removing vCPUs after installation of the guest OS is not an issue with any version of Linux or Windows that's recent enough to still be vendor supported. This warning dates back to the very early days of VMware and is mostly irrelevant now.

In the early days of Linux, though, the kernel had to be specifically compiled with SMP support, and occasionally UP kernels didn't like running on SMP/NUMA systems, or vice versa. Those days are long since mostly forgotten.

These days Linux kernels are almost always compiled with SMP/NUMA support by default and run fine even with one processor. This has been true for all of 2.6 and most or all of 2.4.

Windows has behaved similarly since Server 2003. I wasn't able to quickly find definitive information on the Internet about how 2000 and NT 4.0 behaved, though I seem to recall from distant memory that they may have had issues when switching from a single CPU to multiple CPU configuration.

If you plan to P2V a very ancient system, though, it's theoretically possible you might run into such issues.