When properly implemented and managed, virtualisation means cost savings -- for server hardware, support and administration -- as well as easier server deployment and reduced energy consumption. But for these benefits to come fully to fruition, the virtualisation layer of the stack has to be managed independently of either the application or the physical server.

Fortunately, that's not difficult if you do it right.

Sure, server provisioning and back-up become easier in the virtual world. But monitoring, especially checking the servers' underlying physical resources, becomes even more important. You have to balance the types and numbers of virtualised applications among the physical servers with an eye to best use of resources.

The consequences of getting it wrong can turn some -- or all -- of the virtualised applications running on a physical server into poorly performing slugs.

Successfully managing virtualised servers means understanding that virtualisation introduces a new layer into the server software stack. And you don't manage the virtualisation layer by managing the underlying physical server or the virtual machines or the VMs' applications.

Instead, virtualisation must be monitored and managed separately from the operating system, application or the (physical) server layer. (Note: This assumes you're using hypervisor products like Xen or VMware. There are also virtualisation products like Virtuozzo that virtualise on top of the operating system. That changes the picture somewhat, but the basic principle remains the same.)

Second, you have to keep in mind that while virtual machines appear independent to the applications running on them, they can and do interact through the physical server and its available resources. All the applications and -- to a lesser extent -- the VMs running on the physical server may act as if they're independent, which is part of the charm of virtualisation. But at the bottom, they're all drawing on the same pool of resources from the physical server. A large part of successfully managing virtualised servers is making sure all the VMs have the resources they need when they need them.

The third point flows from the first two. While managing virtual servers is in many ways similar to managing physical servers, they are not the same thing and the analogy stretches only so far.

The good news

Still, despite all the caveats, users agree that virtualisation is a win when it comes to managing servers.

"Virtual servers are a lot easier to manage," says Mike Carvallho, CTO of Radiator Express Warehouse The company, perhaps better known by its 1-800-Radiator moniker, is a chain of 200 franchised radiator shops in the US and Canada. Carvallho manages nine Dell servers that run VMware and are divided between two sites, supporting 55 virtual servers.

Not only are provisioning, capacity management and recovery easier, but there are fewer physical boxes to deal with. "One of the things you immediately lose is all the hardware that was a nightmare to manage," Carvalho says. Among other things, the physical servers tend to be of varying vintages from different vendors, which complicates patches and change management and means keeping on hand a supply of spare parts for the older servers.

When switching to a virtualised environment, many shops take the opportunity to limit the number and types of physical servers, often upgrading to newer technology at the same time. This makes it easier for data centres to keep their hardware in sync by limiting the variety of physical servers in the system.