How many VM can be created per host CPU core?

dangVpham

Hi,

I have a host with 128 cores. How many VM with 4 vCpu can be created with 128 host core?

How can we calculate the maximum VM can be created per CPU cores on a xcp-ng host?

Thanks,
Dang

olivierlambert

There's no limit. It's all about your usage. For Xen, a vCPU is like a process for your regular operating system. You can have more process than physical core, they will be scheduled.

It's exactly the same way in Xen. You could have 500 vCPUs used in total on your 128 cores. This will work well if you don't need 100% of all the vCPUs in all your VMs at the same time. Otherwise, each vCPU will have to wait a bit before getting access to the "real" CPU. That's it!

dangVpham

Thanks @olivierlambert,

How about memory? I have 2TB of RAM on a xcp-ng host, then I will have maximum of 32 VMs or more?

Thanks,
Dang

olivierlambert

Memory is different. You can't assign more memory than the total memory available on a host (so the sum of all VM memory <= total host memory).

If you have 2TiB RAM per host, you could cram around <256 VMs if ALL of your VMs are using 8GiB each.

In real life, you tend to have VMs with 2 or 4GiB RAM, some with 16GiB etc. So the total number is an average depending on your usage.

dangVpham

Thanks @olivierlambert,

Does it apply the same with the pool?

Thanks,
Dang

olivierlambert

A pool is mainly multiple hosts working "together" with a common configuration for network and storage. That doesn't change anything on vCPU and memory for VMs.

dangVpham

Thank you so much @olivierlambert,

That helps me alot with clarification.