Recently I've been using Xen at work. Xen is a virtualising system similar to VMWare ESX it's goddamn awesome! basically you can put as many virtual servers as you like on a single server, running on virtual hardware, the speed hit is a MAXIMUM 8%, usually more like 2% the best bit is, if you are using network/san based storage, you can move a RUNNING virtual server to another physical machine with under 200ms downtime and no interruption in networking.
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