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With the release of Windows 2008 R2, highly available Hyper V deployments have increased in popularity.  One of the big questions during these deployments is what to virtualize and what to leave on physical hardware and in particular when it refers to domain controllers.


Domain controllers are basically highly transactional database servers which service most basic network functions including authentication, name resolution, replication, and of course many secondary services like DHCP or Radius.


In my opinion as long as one domain controller is left on local physical hardware or a remote host, the rest can be virtual.  The reasoning behind leaving one is the Hyper V host servers running 2008 R2 or 2008 R2 Server Core will belong to the domain.  These machines need to function if all the virtual machines are stopped or in a failed state.


As most organizations have more than one domain controller, there should still be a few to virtualize. Lets take a look at the considerations when virtualizing a domain controller.


First the P2V process itself.  Since DC’s are highly transactional, it’s necessary to P2V using offline mode.  This mode will install an agent and a bootable kernel to the machine, then reboot the machine, much like a password reset disk, and begin the process while all of its transactional processes are stopped.  This requires that the hardware be pretty standard so that it’s compatible with the booting kernel especially the network controller.   Once the P2V is finished, the agent and kernel will be removed, the host machine will be automatically turned off, and the virtual machine will be started.  Please make sure to not bring the physical machine back online while it’s connected to the network since this will give you duplicate domain controllers on the same network.


Second is the VM configuration itself.  There are quite a few options when choosing your hardware configuration in a VM.   The type of disk controller, emulated or synthetic, dynamic VHD, fixed VHD, or pass-through disk.  Pass-through Disk actually allows the VM direct access to a host attached disk and is considered the optimal configuration, however it’s not always a feasible solution depending on the environment. For a virtual disk based domain controller, a virtual SCSI controller is the preferred method. Also a synthetic controller is preferred over emulated, however you will need to have the VM guest additions installed before you’re able to create this controller.


VHD type and placement are also important.  Fixed size VHD is extremely more efficient than a dynamic VHD.  Since the size of active directory will undoubtedly grow, a fixed size VHD is the way to go.   Also the VHD should be placed on it’s own LUN or physical disk and shouldn’t share with another VM or the host operating system.  Sharing a LUN between VMs or with the host OS will greatly decrease performance and could have adverse affects on Active Directory.  If the VHD is to be placed on IDE physical storage, make sure that write-back caching is disabled.


So virtual domain controllers aren’t necessarily a bad thing, just remember  Pass-through Disk > Fixed size VHD > Dynamic VHD.

Posted on Friday, November 20, 2009 7:12 PM Windows 2008 Server , MS Hyper-V | Back to top

Comments on this post: To P2V or not to P2V

# re: To P2V or not to P2V
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So is it possible to do a P2V migration and specifying pass-through disks for the new Virtual you create ?
Left by Kragbees on Jun 23, 2010 3:34 AM

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