Visualizing Xen Performance and Utilization

Debian or Ubuntu are both suitable choices for your xen dom0.

As for resources, it's difficult to give strict recommendations because no two setups are the same and ultimately it depends on what your guests are going to be doing.

First up, you won't have all the memory in the physical system available for use in dom0 and your other guest. Xen will take a slice of the memory for itself. The more physical memory, the more xen will need.

The amount of dom0 memory you're going to need depends on the workload it's going to be handling with management of the guests and all the I/0 from the guests. It's difficult to quantify. 512MB may well be enough. In any case you can change if that isn't sufficient.

At a previous job we integrated xen server into our virtualisation platform. Dom0 was set to 768M, which did the job and I didn't come across any instances of this being exhausted by customers. This was on physical boxes with upto 64GB of memory. Another approach is to give all the available memory to dom0, and then slice chunks off each time you create a guest.

That benchmark is only comparing the speed of the native OS to a single guest OS. It is hardly a real-world test. I don't think I would put much weight on it. Most of the KVM camp argues that Xen requires too many interrupts and hops between kernel and user space, but from most of the more real-world benchmarks that I've seen that hasn't really been realized and Xen seems to be a bit faster than KVM.

Sorry I don't have a link to back that up handy. But I will say that KVM is improving fast and seems to be catching up on feature set and stability quickly.

As to which approach is better. The Xen camp will argue that a true light-weight hypervisor is required virtualization to be secure and fast. Xen is also starting to be supported in firmware by some vendors which is also nice. The KVM camp will argue that KVM is simpler and that Linux is capable of being a good hypervisor.

In the end it's still unclear which direction will ultimately win. Xen certainly has a head start and already has a nice market share. But it's not in the mainline kernel, yet. Hopefully that will change soon and there has certainly been a lot of talk about this on the kernel list in the past few months. Red Hat is in the KVM camp now and will be pushing it as the virtualization platform of choice. Red Hat Linux 5.4 which is coming out shortly will be the first to include it. So that will likely attract shops that haven't rolled out or committed to a virtualization platform yet.

As far as tools go, both Xen and KVM use libvirt and QEMU and the tools associated with them. So they share many of the same tools such as virt-manager.

We use Xen at work, and it works well for us. But I've been looking into KVM due to some USB forwarding and PCI passthrough issues I've been unable to resolve with Xen. I'm not sure KVM is any better at this, but I guess I'll find out once I try it. One thing I have noticed in researching my USB issues is that KVM's documentation is more assessable and organized compared to Xen's. But there is no perfect virtualization platform so you'll need to figure out what makes sense for you.