Virtualization of Active Directory Domain Services (AD DS) environments has been ongoing for a number of years. Beginning with Windows Server® 2012, AD DS provides greater support for virtualizing domain controllers by introducing virtualization-safe capabilities and enabling rapid deployment of virtual domain controllers through cloning. These new virtualization features provide greater support for public and private clouds, hybrid environments where portions of AD DS exist on-premises and in the cloud, and AD DS infrastructures that reside completely on-premises.
AD DS replication uses InvocationID and USNs on each domain controller to determine what changes need to be replicated to other domain controllers. If a domain controller is rolled back in time outside of the domain controller’s awareness and a USN is reused for an entirely different transaction, replication will not converge because other domain controllers will believe they have already received the updates associated with the re-used USN under the context of that InvocationID.
For example, the following illustration shows the sequence of events that occurs in Windows Server 2008 R2 and earlier operating systems when USN rollback is detected on VDC2, the destination domain controller that is running on a virtual machine. In this illustration, the detection of USN rollback occurs on VDC2 when a replication partner detects that VDC2 has sent an up-to-dateness USN value that was seen previously by the replication partner, which indicates that VDC2’s database has rolled back in time improperly.
A virtual machine (VM) makes it easy for hypervisor administrators to roll back a domain controller’s USNs (its logical clock) by, for example, applying a snapshot outside of the domain controller’s awareness. For more information about USN and USN rollback, including another illustration to demonstrate undetected instances of USN rollback, see USN and USN Rollback.
Beginning with Windows Server 2012, AD DS virtual domain controllers hosted on hypervisor platforms that expose an identifier called VM-Generation ID can detect and employ necessary safety measures to protect the AD DS environment if the virtual machine is rolled back in time by the application of a VM snapshot. The VM-GenerationID design uses a hypervisor-vendor independent mechanism to expose this identifier in the address space of the guest virtual machine, so the safe virtualization experience is consistently available of any hypervisor that supports VM-GenerationID. This identifier can be sampled by services and applications running inside the virtual machine to detect if a virtual machine has been rolled back in time.
Virtual environments present unique challenges to distributed workloads that depend upon a logical clock-based replication scheme. AD DS replication, for example, uses a monotonically increasing value (known as a USN or Update Sequence Number) assigned to transactions on each domain controller. Each domain controller’s database instance is also given an identity, known as an InvocationID. The InvocationID of a domain controller and its USN together serve as a unique identifier associated with every write-transaction performed on each domain controller and must be unique within the forest.
AD DS replication uses InvocationID and USNs on each domain controller to determine what changes need to be replicated to other domain controllers. If a domain controller is rolled back in time outside of the domain controller’s awareness and a USN is reused for an entirely different transaction, replication will not converge because other domain controllers will believe they have already received the updates associated with the re-used USN under the context of that InvocationID.
For example, the following illustration shows the sequence of events that occurs in Windows Server 2008 R2 and earlier operating systems when USN rollback is detected on VDC2, the destination domain controller that is running on a virtual machine. In this illustration, the detection of USN rollback occurs on VDC2 when a replication partner detects that VDC2 has sent an up-to-dateness USN value that was seen previously by the replication partner, which indicates that VDC2’s database has rolled back in time improperly.
Beginning with Windows Server 2012, AD DS virtual domain controllers hosted on hypervisor platforms that expose an identifier called VM-Generation ID can detect and employ necessary safety measures to protect the AD DS environment if the virtual machine is rolled back in time by the application of a VM snapshot. The VM-GenerationID design uses a hypervisor-vendor independent mechanism to expose this identifier in the address space of the guest virtual machine, so the safe virtualization experience is consistently available of any hypervisor that supports VM-GenerationID. This identifier can be sampled by services and applications running inside the virtual machine to detect if a virtual machine has been rolled back in time.
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