Windows domain controller in a virtual machine: how dangerous is saving its state for a short period of time?

Question

I have a Windows Server 2012 R2 virtualization cluster. All the hosts are connected to an external storage system, and virtual machines' files are stored on external volumes (CSVs). All the hosts and virtual machines are a part of the same AD domain
(mixed Windows Server 2012 RTM / 2008 R2 domain controllers). All the domain controllers are running in the virtual machines on the hosts of this cluster.
To prevent problems when all the hosts are turned off and then on simultaneously (for example, because of a power failure) all the domain controller VM files has been placed on local disks of the virtualization hosts (not on the Cluster Shared
Volumes). As Hyper-V services don't depend on other Windows Server services (except its networking components), it means that my domain controllers can always start, providing the virtualization host can start at all. However, it also means
that those DCs cannot be (quickly) migrated to other hosts while their current hosts are being rebooted. So if I need to reboot a virtualization host to install new updates, for example, I have to shut down the corresponding DC, reboot the host
and wait for the DC to finish cold boot and come back online. It means some interruption of service for our users, which, in turn, requires me to perform the reboots late in night.
The downtime can be significantly decreased by saving the state of the VM in which the DC is running. However, all the articles I've found on the Internet strongly recommend against it. I'm trying to understand why this recommendation was issued in the first
place. However, I'm unable to find a clear explanation. I've found some statements that saving state of a DC can cause serious AD replication problems because of tombstoning, and that the password of a DC computer account may be changed
while the DC itself stays in the saved state, which could prevent the DC from connecting to the domain after its state has been restored. However, those considerations are non-significant when we discuss a short-time
(5 to 10 minutes) saved state.
I work with AD and virtualization long time, and I fail to see any danger in saving state of a DC for several minutes. In my opinion, after its state has been restored it would simply replicate all the AD changes from other DCs, and that's all.
What's your opinion?
Evgeniy Lotosh
MSCE: Server infractructire, MCSE: Messaging

Answer 1

Hello,
as stated in "http://technet.microsoft.com/en-us/library/virtual_active_directory_domain_controller_virtualization_hyperv(v=ws.10).aspx"
Operational Considerations for Virtualized Domain Controllers
Domain controllers that are running on virtual machines have operational restrictions that do not apply to domain controllers that are running on physical machines. When you use a virtualized domain controller, there are some virtualization software features
and practices that you should not use:
Do not pause, stop, or store the
saved state of a domain controller
in a virtual machine for time periods longer than the tombstone lifetime of the forest and then resume from the paused or saved state.
This may sound as it is supported to store it for shorter times and use it.
BUT recommendation also from the Hyper-V Program manager in
http://blogs.msdn.com/b/virtual_pc_guy/archive/2008/11/24/the-domain-controller-dilemma.aspx recommends against using them.
Also best practices
http://blogs.technet.com/b/vikasma/archive/2008/07/24/hyper-v-best-practices-quick-tips-2.aspx
Best regards
Meinolf Weber
MVP, MCP, MCTS
Microsoft MVP - Directory Services
My Blog: http://blogs.msmvps.com/MWeber
Disclaimer: This posting is provided AS IS with no warranties or guarantees and confers no rights.
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