I recently got admin rights to a small vSphere 5 environment (2 physical hosts, ~25 VMs).
The people who set it up didn't have the correct knowledge about CPU core scheduling on the physical hosts resulting in most VMs having 2 or 4 vCPU, even tho they're barely using any CPU power at all.
The VMs are either running Windows 2008R2 or Ubuntu 12.04 LTS. Does anyone knows what the impact on the VMs would be if I reduce the amount of vCPU to 1? (Besides the VMs being less powerfull ofcourse).
If the usage is less you can reduce the vCPU, but I would recommend to keep the minimum Operating system recommendation to do their operation well.
You may have to look for downtime as well, most of the Operating system doesn't support Hot CPU removal
Welcome to the Community,
not sure about Ubuntu, but for Windows 2008 and newer you should be able to reduce the vCPU count without issues, since the HAL is the same for Uniprocessor an Multiprocessor systems.
Though Windows R2 is hot swappable I would not recommend you do that. My best suggestion would be to schedule a downtime or work under current downtimes should you have those planned in your environment. Then power those VM's down and change their vCPU's accordingly. Giving them a small boost in Memory can also increase performance for them while regulating cpu usage if the worry is end user impact. I cannot speak to Linux VM's but in my past those are much more particular when you make back end changes to resources on the fly.
Check these blog which can help you.