I recently had a conversation with a customer around a very interesting problem, and the insights that were gained there are worth sharing. The issue was about VSS errors popping up in the guest event viewer while Hyper-V Replica reported the successful creation of application-consistent (VSS-based) recovery points.Deployment details
The customer had the following setup that was throwing errors:
At the time of enabling replication, the customer selected the option to create additional recovery points and have the “Volume Shadow Copy Service (VSS) snapshot frequency” as 1 hour. This means that every hour the VSS writer of the guest OS would be invoked to take an application-consistent snapshot .Symptoms
With this configuration, there was a contradiction in the output – the guest event viewer showed errors/failure during the VSS process, while the Replica VM showed application-consistent points in the recovery history.
Here is an example of the error registered in the guest:
The big question was: Why was Hyper-V Replica showing application-consistent recovery points if there are failures?
The behavior seen by the customer is a benign error caused because of the interaction between Hyper-V and VSS, especially for older versions of the guest OS . Details about this can be found in the KB article here: http://support.microsoft.com/kb/2952783
The Hyper-V requestor explicitly stops the VSS operation right after the OnThaw phase. While this ensures application-consistency of the writes going to the disk, it also results in the VSS errors being logged. Meanwhile, Hyper-V returns the consistency correctly to Hyper-V Replica, which in turn makes sure that the recovery side shows application-consistent points.
A great way to validate whether the recovery point is application-consistent or not is to do a test failover on that recovery point. After the VM has booted up, the event viewer logs will have events pertaining to a rollback - and this would mean that the point is not application consistent.
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