John Howard - Senior Program Manager in the Hyper-V team at Microsoft

Senior Program Manager, Hyper-V team, Windows Core Operating System Division.

Are VHDs compatible between Hyper-V and Virtual Server and Virtual PC?

Are VHDs compatible between Hyper-V and Virtual Server and Virtual PC?

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This question frequently comes up, but most often is not the question people mean to ask. As written, the answer to the question is yes – there is no change to the VHD format specification used between any of these virtualization solutions. The real question usually is:  “If I build a virtual machine on a VHD in one virtualization solution, will it be compatible with another virtualization solution”.  This blog post focuses on answering the revised question. As you’d expect, the answer isn’t that straightforward.


Let’s start by looking at the obvious cases where the answer is “No, they are not compatible”. Assume you build a virtual machine using Hyper-V, and it is 64-bit. Here, the answer is no as Virtual Server and Virtual PC do not support 64-bit virtual machines.


What about a 32-bit virtual machine built under Hyper-V? Generally the answer is again no. This is because most operating system will detect the presence of an IO-APIC in the virtual machine environment and install an appropriate HAL in the operating system image. Once that image is started on Virtual Server or Virtual PC where an IO-APIC is not present, the image will fail to boot. In theory it is possible to swap the HAL, but not in a Microsoft supported manner (except on Vista and Windows Server 2008 – that’s a post for another day). So the answer remains no. [Maybe an exception would be where an operating system is able to dynamically switch between IO-APIC and PIC modes (this is not the case in Windows Server 2003 or Windows Server 2008 or any OS I’m myself familiar with). I guess that some Linux flavours with appropriate boot switches may be OK, but I’m no expert in this area.]


Now let’s look at some “yes” scenarios. These are the ones where the virtual machine is built under Virtual PC or Virtual Server initially. If you take a VHD image and create a new VM in Hyper-V usng that VHD and simply boot it, yes, it will be fine. If you do not install the Integration Components and subsequently take the VHD back to Virtual PC or Virtual Server, yes, it will also boot fine.


Following on from the previous paragraph, if you install the Integration Components (and let’s stick to Windows Server 2003 for the example), then we swap the HAL to an APIC HAL as the first part of the installer. At this point, the VHD will no longer boot under Virtual PC and Virtual Server.


Hopefully that helps!
Cheers,
John.

Comments
  • PingBack from http://msdnrss.thecoderblogs.com/2008/02/28/vhd-compatibility-between-hyper-v-and-virtual-server/

  • Honestly I was shocked when I read John's recent post about " Are VHDs compatible between Hyper

  • Honestly I was shocked when I read John's recent post about " Are VHDs compatible between Hyper

  • There is one particular scenario where you could be faced with this when booting a VM using Hyper-V.

  • Honestly I was shocked when I read John's recent post about " Are VHDs compatible between Hyper-V

  • I hope that this compatibility issue can be updated to include the ability to remove the "Integration Components" from any state of Hyper-V virtual machine so it could be installed on a MS Virtual PC (2004-2007) or MS Virtual Server 2005 R2 version; once it gets out of Beta.

    As I am on the subject of dream requests, being able to support conversions from Physical to Vitual with a seemless tool like VMware's "Converter" utility would be a lot less painful than the Microsoft route.

    Oh and heck, why not ask for a solution only the uterly insane might request, compatibiliy with actual VMware virtual machine files; or perhaps a means to convert VMDK to VHD integrated into the non-Beta version of Hyper-V?

    One can always dream.

  • Even the straight migration back and forth between Virtual PC 2007 and Hyper-V does not really work. I have a MOSS development environment running under Virtual PC 2007 with Office 2007, etc. When I boot it under Hyper-V it tells me the hardware has changed significantly and that I have to re-activate Windows. This is also true of SharePoint Designer 2007 and possibly other Microsoft applications. Having to re-activate the OS and Office apps every time you boot it between VPC2007 and Hyper-V does not seem like a reasonable option to me?

  • Success!

    I made W2003 R2 SP2 Std Edt 32bit VM under Virtual Server 2005 R2 SP1, migrated VHD to Hyper-V, installed Integration Services, created new machine, et voila! It worked well.

    Then I was asked to move it back to Virtual Server 2005 R2 SP1. I've tried with Uninstalation of Intergation services, but didn't work. Then I mounted VHD as drive letter and replaced HAL.DLL with old one. IT WORKS !!!!

  • Nous voici donc à la troisième étape de notre prise en main d'Hyper-V sur une configuration

  • I appreciate the article.  I've just posted a recent How-To on the conversion side.  Hope this helps.

    http://www.groovypost.com/howto/microsoft/windows-server/migrate-microsoft-virtual-server-2005-r2-vm-to-windows-server-2008-hyper-v/

  • Ja det går alldeles utmärkt att boota en VHD fil som du skapat med Virtual PC eller Virtual Server i

  • Lately it's been very quiet on my blog. There are a couple of things to that. First and foremost there

  • Any news of coverting from Hyper-V to Virtual Server an Virtual PC? I have developers needing this possibility from time to time? I have heard that converting x86 VHDs with WSS or MOSS (SharePoint) could give problems, any comment's on that one and I would appreciate it?

  • Morthorst - no, I've never heard of that (MOSS/WSS going TO Hyper-V). Can you provide a pointer to something for me to investigate?  The other way back can be problematic as I discussed above. I would recommend if that is required to keep a backup of the original VHD and configuration under Virtual PC/Virtual Server.

    Thanks

    John.

  • "I guess that some Linux flavours with appropriate boot switches may be OK, but I’m no expert in this area."

    Linux do not use HALs and instead include code for all combinations in the kernel, thus it should always work.

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