Quoting EnterpriseNetworkingPlanet

Whichever way you look, appliances are multiplying like rabbits. Vendors of security, network management, and many other types of products are offering easy to install appliances as alternatives to software packages and increasingly they are phasing out their software offerings to concentrate exclusively on appliances.

The attraction of an appliance is easy to understand. Procuring and building a server, installing the software, finding a place for it and connecting it to the network and a power supplier can be a time consuming and expensive task. By contrast, an appliance can be unpacked, plugged in and switched on in less than an hour.

But virtualization technology offers a real alternative to appliance sprawl: virtual appliances. The thinking behind them is simple: since virtualization enables you to decouple servers running applications from the physical hardware they are running on, why not decouple an appliance from the physical appliance it is running on. Why not run multiple virtual appliances on a single physical appliance?

And since appliances are generally standard servers made by Dell, HP or some other well known vendor, the physical appliance used to run virtual appliances can be any of the servers in your data center – which may already be hosting a number of virtual servers running business apps or network services.

To create a virtual appliance – perhaps a firewall, a security gateway or even a router - a virtual server is created and the appliance's hardened operating system is loaded to create the virtual appliance. This is then loaded with the appliance's software, and the resulting virtual machine file is ready to run. It works just like a physical appliance and can update itself automatically just as a real appliance would.

There are other benefits to virtualizing appliances beyond ease of deployment, or course, including hardware consolidation and reduced power and space requirements. Perhaps the biggest, says John Humphreys, a program director at analyst IDC, is cash savings through simpler administration. "Most customers get into virtualization to lower hardware costs and drive up server utilization, but it also drives down operational costs. Less time managing physical infrastructure can lead to big cost savings."

And think what happens if a traditional appliance fails. You're in trouble, for a short period of time anyway, until you can get hold of a replacement unit to switch in. Unless of course you decide to pay for a spare appliance and keep it in a cupboard, ready for just this eventuality. But if your appliance is virtual, you can move the underlying virtual machine file to run on another server – almost instantly, in some cases – or simply create a new instance of the virtual appliance and put it to work on a working server straight away. Since you are only ever using one virtual appliance at once, you should have no licensing issues, and of course the server is less likely to fail in the first place if it's a high end, fault-tolerant machine. Which it can be, if you want it to be. With a physical appliance you get what you are given, but with a virtual appliance you can decide exactly how fault tolerant the server it runs on should be. If your organization has strict rules about what hardware you use and where it can be bought from, for security or any other reason, then you can have the best of both worlds – the appliances you want, running on your company's approved hardware

The ability to spin up a virtual machine also has its benefits when it comes to testing appliances. Rather than arranging a trail and waiting for a box to arrive, plumbing it in and trying it out, it's much more convenient to download a virtual appliance, put it to work on a test box, and then delete it (or buy it and switch it to another server) once you've put it though its paces.

Are there any disadvantages to virtual appliances? Like all virtual machines there is some virtualization overhead so they are will run slower than physical appliances, but this may not be a problem for devices which carry out background monitoring duties, and in any case virtualization overheads are going down rapidly as hypervisors make use of hardware assisted virtualization. In some cases, though, poor performance could be an issue.

And in terms of costs, it's likely that many vendors will charge the same for a virtual appliance as a physical one, meaning you'll effectively be landed with the hardware costs. But since the whole point of virtualization is that virtual machines can share physical servers, you may not need to buy additional servers to run the virtual appliances you want – you can squeeze them on to existing boxes. Virtual appliances can be quite small, so this is a realistic prospect, but if they are network intensive they may not happily coexist on the same physical server.

The number of virtual appliances available to run on VMware and Xen virtualization technology is large and growing, and looks set to increase as their popularity catches on. The days of the standalone security or network management appliance may be numbered.

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