Today a co-worker called me and told me they where testing something out and they stumbled on a little issue. Here’s what they did, they where trying to start 125 VMs at once on an ESXi 4.1 host to do some load testing.
This al works just fine when they use a single vCPU for each virtual machine, they could start all 125 VMs at once, no problems at all.
However when they where doing the same test but now with an extra vCPU (so in total they assigned 2 vCPUs for each virtual machine) they couldn’t start more than 100 VMs.
The other 25 where unable to start and the received a error message that the system could not power on the VMs (look at the screenshot below).
Weird? well not really, especially if you look at the error description. You could already guess what’s going wrong here, it looks that they have reached some limit in the amount of used vCPUs on the ESXi host.
To solve this little issue you need to change the maximum number of vCPUs to for example 512 vCPUs.
To increase the maximum number of vCPUs to 512:
- Launch the vSphere Client.
- Click the host.
- Click the Configuration tab and then go to Advanced Settings.
- Go to Vmkernel > Boot > maxVCPUS.
- Modify the parameter to 512.
Check out this VMware KB article to find other ways to change the maximum number of vCPUs.
Thanks, goes to Krishan Khoesial who has worked this issue out.