How many vCPUs per Physical CPU can we use?

Station-Already

Hi all - I just had a question concerning vCPU to CPU mapping. At the moment, we have a couple of older servers in a VMware cluster, and we're considering a migration to XCP-ng because a.) open source, and b.) VMware's pricing is outrageous for our purposes.

At the moment, I suppose I'm "overallocated" if one vCPU is equivalent to one logical CPU core (and on our current cluster, none of the nodes have CPUs with symmetric multithreading capabilities). Despite this, the cluster nodes themselves report REMARKABLY low CPU utilization - most likely because I'm a BIT generous with my VMs, but our company is quite small so they aren't pushed that hard.

Is this something that XCP-ng is capable of handling similarly? I mean, I'm obviously not going to go insane allocated CPUs that aren't there, but a little bit of wiggle room vis-a-vis 1 vCPU to 1 Logical CPU would be appreciated, and I'm wondering how XCP-ng handles this.

Thanks!

gskger

@station-already Maybe the post CPU question about core numbers by Olivier can give you some ideas?

olivierlambert

Those are good reasons for migration You are not the first one to go that route! Hope we'll be able to assist you the best we can, via our community or pro support, depending on your choice
Yes, that's correct. Xen will schedule CPU time between VMs. If you use a ratio of 2 vCPUs per physical core, then you'll have half of the CPU time for each VM (if all VMs are at 100%). In real usage, going for 2 virtual to 1 physical is OK. More than that is also OK if you don't use a lot of resources. Less is better if you need more constant firepower.

Station-Already

@olivierlambert That is wonderful news. We actually will be closer to a 1.2:1 or 1.1:1 ratio of vCPU's per physical core - our hosts are old beasts, but we don't work them hard. We had some 10 or 12 VMs on one while it was running VMware (DURING our migration, so a it was running more than it normally did) and it was consuming maaaaaaybe 30% of the available CPU power. There really aren't that many of us.

We are probably not going to purchase XCP-ng Pro Support, but we absolutely WILL be purchasing a Xen Orchestra subscription at the Starter tier. I think it hits all the features we need, it gives us a real support contract for our virtualization solution, and it is our great honor to support an open-source project!

Thanks for answering my question. Hopefully it's helpful in answering someone else's out there on the internet.