During a fail over DR cases when the Main site is completely not available we need to carry over few steps to activate Exchange Services according to the type of DR setup we have.

Sequential steps needs to be carried over in terms of  restoring the DAG,activating the DB’s on the DR site pointing the exchange DNS records to the DR site ip’s.

Failover scenarios varies according to the namespaces, no of sites in Exchange :

UnBound Name Space- Single name space for all Exchange URL’s for both the main and DR sites which is best recommended.
Bound Name Space – Very complicated and not recommended since we need to use seperate URL’s for Main and DR site.

If we have a three site setup with FSW in third site or if the FSW is placed in the Azure directory in the 3rd site then no manual activation of the database copies on the DR site is required. Only exchange DNS job on the DR site is required.

For detailed information on DAG DR setup i have written a previous blog which can be referred:

https://exchangequery.com/2016/05/04/dag-in-exchange-2016-and-windows-server-2012-r2/

From Exchange 2013 the Dynamic Quorum in the failover cluster adjusts automatically and recalculates the active nodes if its on a sequential shutdown for a two site setup.

During a DR activation in the DR site when the main site is completely not available after rebuilding the DAG cluster on the DR site we might come across the below error for some databases

In my test case it was the below:

Stop-DatabaseAvailablityGroup – for the Main site completed successfully with no errors
Restore-DatabaseAvailabilityGroup – completed successfully except some warnings for one mailbox node on the DR site.

On the server with warning noticed that all the DB’s were in failed state.Tried to mount them and got the below error

An Active Manager operation failed. Error The database action failed. Error: The database was not mounted because its experienced data loss as a result of a switchover or failover, and the attempt to copy the last logs from the sourcserver failed. Please check the event log for more detailed information. Specific error message: Attempt to copy remaing log files failed for database DBNAME. Error: Microsoft.Exchange.Cluster.Replay.AcllUnboundedDatalossDetectedEeption:

By looking into the above message its very interesting to see that the DR site DB’s are trying to reach the Main site copies to the get the information though the DAG cluster is activated on the DR site and the PAM is on the DR.

The below command can be used just in case if the DR copies are not mounted after activating the DR site DAG.

Move-ActiveMailboxDatabase “DBNAME” -ActivateOnServer DRMailboxServer -SkipHealthChecks -SkipActiveCopyChecks -SkipClientExperienceChecks -SkipLagChecks -MountDialOverride:besteffort

So we need to be very clear that this error will not occur normally until and unless there is some data loss for any DB’s during the DAG DR activation.

Usually when we do a Restore-DatabaseAvailabilitygroup on the DR site all the DB’s should be mounted on the DR site.

The above command can be run only if the database copies are in a failed state after DR site activation and if they are not getting  mounted.

Thanks & Regards
Sathish Veerapandian
MVP – Office Servers & Services