VMware co-founder Mendel Rosenblum has aired his views on last month's Citrix-XenSource acquisition, on what features the CPU vendors should put in their products in support of virtualisation, and on the brouhaha following his remarks at LinuxWorld, which were widely reported as predicting the death of the operating system.

In an exclusive interview with Techworld, we discussed these and other issues with Rosenblum at VMworld in San Francisco, the virtualisation industry's conference. This is an edited version of that conversation.

Q: Last year, you said that virtualisation "doesn't imply the death of the OS but it's a real opportunity for someone to do OSes for appliances." Yet at LinuxWorld last month, you were reported as prophesying the death of the OS. What changed your mind - and why? A: Journalists like to put things in headlines that I actually didn't say. I was consistent but was surprised at the press response.

The point was that once you have the virtualisation layer driving the hardware, that's taken away some of the functionality of the OS, which gives the OS a chance to focus on its other major goal which still very much needed in the application environment. So where modern OSes have gone is to control hardware and the application and are running into a dead end - that's where the controversy came up. As for your quote, I still agree with that.

Q: There doesn't appear to have a lot of emphasis on virtual appliances it at this event. Is it still important? A: This VMworld it wasn't pitched as much, but I find it encouraging that some of them are very compelling from end user perspective and enterprise infrastructure - there are firewalls and this kind of thing.

Some of these companies are excited because they can abandon the 1U box and sell the software to customers - it's a much better way of doing business.

Q: What do you think of the Citrix/XenSource merger from a technological perspective? Does it make sense? A: If I were in Citrix's shoes would I have gone in that direction? There is a layer to control I can see it from that point of view as they've long suffered from being on top of Microsoft.... I can see why they might do it but there are a lot of challenges.

Q: Given that there are now so many different approaches to virtualisation, some of which are gaining traction, why wouldn't VMware change its hardware virtualisation-only approach? A: Only thing we've ever said is that we're not going to build hardware. I can see that there's application level virtualisation has advantages but doesn't solve all of them. It won't replace what we do but could augment it. Our customers have discovered that they can run our VMs and use Softricity to inject the applications, and that looks like a really nice combination.

Q: What about OS level virtualisation? A: That's an interesting one, I'm less optimistic, it does some things really well, you can get a lightweight VM but you inherit a lot of the problems OSes have, you're dealing with a complex things, then there's compatibility. Fundamentally, I believe you want something at the lowest level just to manage the hardware and if you do that I'm not sure you want something at a higher level. But I wouldn't claim that VMware would never do that.

Q: Isn't VMware duplicating what the OS is doing when it comes to I/O and doesn't that mean a double hit on I/O? A: Most well-designed hypervisors don't cache things so when an application writes you write it out. But there are problems with extreme resources where an OS starts paging and then the hypervisor might then doing, but we have ways of dealing with that to eliminate that problem.

Q: Can hardware support eliminate the I/O performance problem? A: Hardware assist gives us native speed performance and they won't be running any faster than native speed. Doing it in the OS you can make it lighter weight you might have resource advantages.