Symptoms
Windows Server 2012
When backing up a disk on Replica machine, Replication health goes to warning/critical and replication goes to Error state on primary server. Additionally, during the backup interval the following events are logged in the Microsoft-Windows-Hyper-V-VMMS-Admin log:
On Primary machine:
On Secondary machine:
Windows Server 2012 R2
On Windows Server 2012 R2, the replication continues without any state change and replication health will be normal. However, the following events are logged on the Secondary server:
Cause
This happens when you take backup of the disk on recovery host which has data related to replica VM. Both backup and replication acquire modifying lock on VM to complete their operations. At any point only one process can operate on the VM.
For example: If a user starts backup, it acquires the lock on VM and doing so will make replication fail during the time back-up is going on. Depending on the number of cycles missed, replication health of the secondary machine changes to warning or critical and primary machine replication state goes to error state.
In Windows Server 2012 R2, Delta replication means only sending changes. Apply of change log is done asynchronously. This means that RPO of replica VM will be intact even when backup is going on. Hence you only see errors in event logs while replication health and state will be normal and replicating respectively.
Resolution
In Windows Server 2012, Replication automatically succeeds without any errors after back-up finishes. If back-up takes too long to complete, replication may go into Re-sync required mode. Check the replication health on the replica VM. If it says, “Resync Required”, click on “Resync” on replica VM.
For more information on Resync, refer to the following blog:
Resynchronization of virtual machines in Hyper-V Replica
In Windows Server 2012 R2, replication continues normally. No explicit action is required from the user end. If user initiates Fail back after backup is complete, all the pending changes will be applied if user selects latest recovery point before Failover starts.
The following Powershell command will show the latest time at which changes were applied on Recovery side in case of Windows Server 2012 R2:
measure-VMReplication | select Name,LReplTime
