This article discuss about the Domain Controller dilemma.  The basic problem is this: if you decide to virtualize all of your servers, how do you handle the domain controllers which control the domain used by your Hyper-V servers?  There are a couple of options that you can consider here:

Keep the root domain controller on physical hardware: By keeping the root domain controller on separate physical hardware you can avoid any potential for problems.  However you also miss out on the benefits of virtualization for your domain controller (better hardware utilization, hardware mobility, easier backup, etc...).

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