Thursday, February 16, 2012

DR Recovery of a Hyper-V guest using Virtual Server 2005

Here's the scenario: You've convinced your managers to try out virtualization, and you've decided to buy a single server along with some shared storage, leveraging your current server as a second host for your environment. You've made the choice to try Microsoft Hyper-V (after all, it's included with Windows 2008 R2), and you get everything going on the new host. But when it comes to putting Hyper-V on the current server, it turns out that the CPUs, chipset and BIOS are incompatible. After some research, you learn that it will support Virtual Server 2005 (VS2005).

Microsoft is using the same virtual hard drive format (VHD) that they inherited from Connectix, so on the surface, one would hope that it would be possible to use a guest's VHD in either environment: simply create a new VM with the VHD as the primary disk and boot. You won't be able to use any of the really powerful features of virtualization like Live Migration, but at least you'll have an opportunity for short DR RTO/RPO if you could readily move the guest between hosts—even if it's a manual process.

Unfortunately, the two Microsoft hypervisors have very different VM (ie, virtual hardware) specifications (and limitations), and essentially share only the VHD format in common: attempting to boot a Hyper-V guest in VS2005 will fail miserably, and may do so without any indication of why it's failing: you get stuck at a black screen, and that's it. No BSOD, nothing.

Fortunately, there exists many references to porting Hyper-V guests into other environments that are just a Google search away. All those resources tell you that it's possible to run the guest in Virtual PC (the desktop hypervisor, not the server hypervisor) if you remove the integration tools. Some of them also add the step of performing a hal.dll swap. Of course, one thing is absolute: the version of the OS that you're trying to migrate must be a 32-bit OS: while Virtual Server 2005 itself may run fine on a 64-bit OS, it cannot support any 64-bit guest.

Fine. Caveat emptor. We didn't virtualize a 64-bit guest with Hyper-V, so this just might work...

Here's the first problem with the porting technique: if you could get the guest running on a recovery host in order to remove the integration tools, why would you fool around with doing anything else? It's running. Stop fooling with it and move on to get your production hypervisor working again!

Luckily, the real trick isn't related to removing the integration tools, it's getting the right HAL.DLL on the guest. So here's how that is done.
  1. Prepare your Hyper-V guest
    1. On your VS2005 host, create a VM running the same OS as the Hyper-V guest that you wish to recover. This is only temporary, as you're after one special file from the guest.
    2. Get a copy of its hal.dll (%systemdrive%\WINDOWS\SYSTEM32\hal.dll)
    3. Put a renamed copy of the VS2005 HAL (eg, hal.dll.vs05)  on the production Hyper-V system drive, in the same folder as the original HAL.
  2. Test your disaster
    1. Copy the Hyper-V guest VHD to your recovery host. If you do it while your guest is running, it will reliably reproduce the "improper shutdown" that will occur if your Hyper-V host dies unexpectedly.
    2. Mount the VHD as a local disk on your VS2005 host machine; if you're running the latest version, this capability is part of the package.
    3. Rename the original hal.dll to something else (eg, hal.dll.hyperv) and rename your VS2005 HAL to hal.dll. By renaming—instead of replacing—you should be able to reverse the process when you have a production Hyper-V host available again.
    4. Dismount the VHD
    5. Create a new VS2005 guest, using the copied, modified VHD as the primary disk. Do not connect the network to the guest, or you'll have all sorts of problems, the least of which being errors about duplicate names on the network.
    6. Boot the guest. If you get all the way to the logon prompt, you've succeeded.
  3. Test recovery
    1. Shut down your test VS2005 VM.
    2. Mount the VHD as a local disk.
    3. Reverse the hal.dll renaming operation completed in 2c.
    4. Dismount the VHD
    5. Move the VHD back to your Hyper-V host
    6. Create a new Hyper-V guest using the VHD from the VS2005 system. Again do not connect the network to the guest, or problems will ensue.
    7. Boot the guest. Again, if you get a logon prompt, you're golden.
What to do if this doesn't work: go back to management and get a second, new server. Your company's livelihood shouldn't rely on a "bailing wire & duck tape" solution like this. You've already got the shared storage; that's the most expensive part of a highly-available virtualization environment.

What to do if this does work: see above. Don't rely on this solution. This is, at best, a complete kludge, a pig in a prom dress. I'm not even sure that you'd get support from Microsoft if you had an issue, even if it was totally unrelated to the environment.

And in my (biased) opinion, use VMware instead of Hyper-V. While both products have Type-II hypervisor options if that's a requirement (you need a full Windows OS on the "bare metal" host), the VMware guest is a much more mature, portable virtual hardware platform (it really can be as easy as copying a file when you're ready to move to the latest versions of VMware); Microsoft (and all the competition, for that matter) are still working to reach the sophistication & reliability of what VMware introduced years ago.

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