Virtualization

I've been exploring virtualization technology for a year or so. At first I was interested in checking out Xen (since it was open source). But it appeared complicated and I didn't want to get involved in a compilation/setup cycle just yet. Recently, I've had the chance to actually try out a few products and here are some of my experiences with it.


I've tried VMware workstation in the past. It works well and it allows you to explore working with VMs on your own desktop. My problem with it is that when given a choice of working through a VM versus through the host you would choose the host naturally. I wanted a hypervisor (or a light-weight OS) which would not distract me from working with the VMs.


So I downloaded and tested different hypervisors available. XenServer and KVM. There is also VMvisor from VMware but I figured it was similar to XenServer. They are all basically the same.


XenServer and VMvisor are hypervisors which means they are light-weight OS managing and running on top of physical hardware. They provide the resources for running VMs on top of the hypervisor. I installed XenServer first. It was an easy installation on my workstation. After installation you have a curses interface to the OS.


But I didn't know how to create VMs. I searched for the documentation online (looks like good documentation) and I discovered that I have to use a client to access the VM manager to create VMs. So what I was losing in deciding to go with a hypervisor was the use of a desktop. The hypervisors are for servers or any headless system that will run the software. Losing the use of my new monitor wasn't part of the plan. So I decided to ditch XenServer.



I moved on to KVM. KVM stands for Kernel-based virtual machine. KVM was the chosen virtualization technology for Ubuntu. Xen technology was purchased by Citrix systems a while ago. Xen is still provided as open source but the enterprise product is requires a license. So I wondered why KVM for Ubuntu? The reason as provided by the KVM site is that the development effort was minimized because it utilized the Linux kernel. Why build a light-weight OS with scheduling of processes and workloads when there is an excellent open source OS available already? So what KVM does is it adds a module to the kernel to trap whenever the kernel encounters certain instructions. Oh yeah, KVM assumes you have virtualization capabilities provided by the CPU. So I tried it.



It took some time but I enjoyed playing with KVM. It's an interesting set-up. And the documentation is Ok. KVM by itself only allows you the capability of running VMs. But to run a VM you require some sort of emulator. So how this solution works is that you require either VMware player, Xen, or QEMU. In other words, KVM turns your Linux host into a hypervisor. The good thing is that you still have the desktop (GUI) where you can run VMs. The difficulty is that there are extra software layers involved that makes the solution less than simple. I.e. libvirt. Libvirt is a library which permits access to VMs. Libvirt isn't necessary of course. All it does is gives you a standard front-end to your VM solution. So KVM => Libvirt => emulator.

What's cool about Libvirt is that you manage the VMs via a shell console (virsh) or a gui console (virt-manager). Anyway, I enjoyed working through the solution.



I didn't settle on KVM (although I'd like to). It has alot going for it. Principally, it's open source and second it doesn't shield you from it's capabilities. The only drawback is it's complexity. It took a little time to get started. But the reason I went back to VMware workstation is the performance of QEMU. With non-Linux OSs QEMU just didn't perform very well. I installed Windows XP and Solaris 10 using QEMU and it was slow. Very slow. Plus the fact that converting a QEMU created VM (i.e. qcow2 format to vmdx format) didn't necesarily mean that I could take it to a VMware player and play it back. I could have tried KQEMU but it was another level of complexity that was eating up more of my time. So I'm back to using VMware.



Long story short, I would prefer to use KVM/libvirt/QEMU if QEMU performed better out-of-the-box. I would also prefer to have a document which described the architecture of the solution to ease new users into using these technologies. Finally, based on some of the documentation I researched, it opens up clustering and cloud computing which I'm also interested in exploring. So it has alot of interesting technologies going for it. But for my current purposes it's complicated to setup and maintain. I hoped to use both KVM and VMware at the same time but it's not possible as both KVM/VMware add a module to the kernel to run virtualization. So they are mutually exclusive.