The VMWare ESX and Xen hypervisors are basically cut down Linux kernels, HyperV is the core elements of the Windows kernel.  It is a myth that "there isn't anything but hardware", otherwise why would you need a hypervisor at all?.  Sure, having the rest of the shell and gubbins you get with Ubuntu adds some size to the base image, but that doesn't affect performance noticably on hardware that allows native virtualisation anyway.  Also, unless you are network booting your host and having it diskless, all the storage is usually remote anyway.  Having a proper amount of OS does let you do a lot to monitor the actual hardware of the VM host, for example running network host monitoring agents on the host (rather than the guest, which may not be on the same host from one moment to the next).

If you *really* want to, you could custom build a Linux based VM host distribution that does all the above in a footprint equivilent to the ESX component of vmware, but for the reasons above why bother?

Courtesy of kvm and libvirt, the guest OS *can* talk to hardware directly, unlike all but the most recent version of ESX where hardware (device) calls go through the hypervisor.  There are benefits and downfalls to either approach, but at least you get to choose.

Kernel updates to Ubuntu would be handled in the same way as the regular ESX updates that you should do, or hardware upgrades and maintenance: move VM's off that machine onto one of the others, then reboot that machine.  After all, one of the main points of virtualisation is you can maintain the hardware and hypervisor platform independantly of the VM's themselves... so you always have at least enough hardware capacity to cope with one physical machine being offline.  You can ripple that across servers as each one goes down and up again.

We actually use Debian as our host OS, with mostly Debian and a couple of Windows servers as guests.  With RAIDed 72gb disks on each host we have about 70gb disk free, and the backend storage (iSCSI) contains all the virtual machines.

VMWare would certainly dispute that their ESX product is a cut-down Linux kernal. I was at a conference last year where the very issue was raised and VMWare said that although the hypervisor foundations lie in Linux, the hypervisor is very different to what you and I would know as Linux.

The argument for running a very thin hypervisor (performance aside) is security. It follows that the smaller the hypervisor foot-print, the less code there is and therefore less chance of bugs and security holes.

Thank you ictstbenedicts, yes, as you say ESX is very different to a standard Linux kernel, and what I said was an over simplification.  What I was referring to was this:

http://www.theregister.co.uk/2007/08/16/vmware_derived_from_linux/

The thing is, we cannot prove ESX is any more secure than a regular Linux installation, because we don't have access to the source code.  Peer review is ultimately the only way to have a chance at certifying security (military organisations require access to source code  for the software running their jets for this very reason).

One could argue that you could feasibly gain more security from a properly configured Linux based distribution, as you can run local security tools on the host to compliment anything you have in the network layer (firewall on the local virtual-ethernet bridge, confine guests to a vlan at the host layer, not on the guest or switch, host and network intrusion detection tools).  This assumes your additional tools are similarly well peer-reviewed and secure.

Just to let the forum know of some developments.

DNUK have recently updated Openvirt to standardise on Ubuntu 9.10 Server as the base OS. This brings some significant benefits in terms of virtualization implementation which we feel warrants a new point release at this time. We anticipate several improvements can be made over the coming months but are certainly looking to put out a further upgrade to Openvirt in around May 2010, to coincide with the release of the next LTS version of Ubuntu.

For any organisation interested in trialling Openvirt in order to assess its suitability for your requirements, DNUK have decided to make the platform available for a one-off charge of £75+VAT. This entitles the purchaser to one instance of the software itself plus one hour of our Pro-Support in order to get the platform up and running and begin tuning it to your requirements.

Further support can be purchased at £75+VAT per hour as necessary and users have the option to use community/self/3rd party support in addition to ourselves. We hope with this development, DNUK will be able to provide organisations looking at virtualization projects with a low cost option to trial a system, which we believe can help make huge savings over other available platforms.

Please visit our website for more details http://www.dnuk.com/solutions/virtualization/virtualization-openvirt.php and contact us for any further information you may require.

Whilst you are there, take some time to look over our other systems and solutions, designed to use open source components to completely remove vendor lock-in. We'd rather you came to us because you want to, not because you have to.

-Chris Williams