In other words, you must empty the host before add it to the primary pool, of course, you can migrate VM from one pool to another. And if your hw is not the same, but similar, you'll have an heterogeneous pool.

Everything clear.
Thanks.

I'm asking because I need to increase my pool's CPU resources, the easy way is buy the same processor, but thinking in a more powerful option, RAM and networking features are ok, but compute performance is being starvest by some VM.

I have an old pool, which I had to upgrade some years ago, but no more original model components were available, so I had to buy a new model, new cpu generation, so it became an heterogenous pool. The problem was not the lost of new cpu features, the problem is that time to time, when I performe a live migration VM, it hangs and I must reboot the VM.

Keeping in mind to avoid and heterogeneus pool, I think the safest option is 5220S, because 6226R implies more changes which I unknow in the architecture, maybe new features, that's what I unknow.

Any piece of advice?

I think no, but I'm wondering your opinions.

In my pool I have 5 server with double intel 5217 (8C 3GHz) processors, I'detected that CPU start to be very higth , so thinking in more powefull CPU. Model 5220S is 18C cores at a similar speed. Both, 5220S and 5217 are gold series of "cascade lake" architecture. So, features must be the same, only cores and speed differ. Intel 6226R is another interesting option, quite similar speed and doubles chip cores.

According to citrix, "Combining different CPU models that have identical features" is the 4th type of heterogeneous pool, and "Type 4 represents CPUs with different marketing model names but identical model number, family, and feature flag attributes. Because they have identical attributes, these combinations have always been supported and do not require a mask, but their compatibility has not been obvious when comparing the marketing model names."

Do you think these chips (5220s and 6226R) would mean an heterogeneous pool or not?

In other words, you must empty the host before add it to the primary pool, of course, you can migrate VM from one pool to another. And if your hw is not the same, but similar, you'll have an heterogeneous pool.

5% sounds too much for me, but I'll check it in some populated pools (with host's cpu below 80%) to have a better idea.

Maybe somebody could point some metrics to predict future cpu/Network overcommit.

As always, reading you expands my knowledge. point 1, perfect, as I thought it was. Point 2, I thought dom0 talk to xen hypervisor by XAPI, thanks for your correction, I supposse dom0 is the only one that can communicate with xen and xcp-ng xencenter or XO use dom0 via XAPI, beeing dom0 who finally passes the order to xen. Point 3, that's what I want, observe parameters like wait and st from top command to detect bottleneck or overloaded resources, like CPU or SR access.

And going back to my original question, what st values should be allowed?. for instance, 20% must indicate a serious lack os resources in the VM.

Anrother wrong concept here for me or I've explained myself wrong. Xen is the hypervisor, is not dom0 a hidden vm where you connect when ssh to the host?, where xe commands are played and the one who controls the rest of the VM running on the same host? Like xencenter, xcp-ng center or XO, dom0 is a way to give orders and get states to/from xen hypervisor via XenAPI.
For instance, start or shutdown any VM is a order that dom0 gives to xen hypervisor, or at least it's what I've always understood.

When I said that I was looking in top, I mean the top command in the VM, not the dom0

Hi everyone.
As far as I'm concerned, the st parameter or steal time is the % of cpu the hypervisor (Domu0) steal from the VM, because it can't supply enough time slots due to a overcommited host. When the host is empty and has plenty of resources, it may give as much cpu time as asked.
I don't know if DomU0 works in extrict or best effort mode, giving just the resources aplied or even more when plenty of resources.

My question is, keeping in mind that hypervisors are not ideal, what % of st is allowable when the host is relaxed?, because I observe st values below 1% in a VM on am empty host.

I'm going to populate the host in next days, and a bit worried about not overcommit it, so I'll be watching this parameter.

As always, if I'm wrong in some point, please correct me.

Well, for me was not clear untill your current answer. As you can see, my conclussions were wrong, but that was just a suggestion.

Could I suggest to update https://xen-orchestra.com/blog/xen-virtualization-modes/ with your answer as the current state-of-the-art?
Do you want me to post my question to get your answer? It's clear for me now.

Another solved post

Regards