The transaction logs are not flushed even after a successful Full backup of Exchange Server 2010 (and higher) DAG

Problem

Even though the Exchange Backup DAG component is correctly selected, the backup job option "Information Store --> Backup Method" is set to Full-Database & Logs (Flush committed logs) or  Incremental Logs (Flush committed logs) and the job finishes with no errors, Exchange 2010 (and higher) does not flush the logs.

Cause

Exchange Server is responsible for flushing the transaction logs after a successful Full Backup as specified in the Microsoft documentation:

Exchange "Exchange Full Backups"
http://msdn.microsoft.com/en-us/library/aa579136(v=exchg.140).aspx

Exchange "Differential backups"
http://msdn.microsoft.com/en-us/library/aa579157(v=EXCHG.140).aspx

Exchange "Incremental backups"
http://msdn.microsoft.com/en-us/library/dd877022(v=EXCHG.140).aspx

Exchange VSS Writer..Interaction Scheme
http://msdn.microsoft.com/en-us/library/aa579076(v=EXCHG.140).aspx
 

In an Exchange Server 2010 (and higher) DAG environment there are two writers: "Store Writer" associated to the "Information Store" and the "Replication Writers" associated to the Exchange Replication service when DAG is configured.

The writers can delay the deletion of the logs after a full backup if there are pending operations which affect the logs. For example, if replication among the "Mailbox Exchange servers" is disabled or is not working properly, the logs will not be flushed.

Log truncation doesn't occur on the active mailbox database copy when one or more passive copies are suspended.All the Exchange Database copies should be active and replication should happen for transaction logs to be flushed

Solution

Set the Exchange Writers logging level to "Expert"  so that Exchange Server reflects every detail of the Writers operation. See https://technet.microsoft.com/en-us/library/dd233256.aspx for details

a) Check that the Replication is working properly among the Exchange Mailbox servers.

b) To prevent the log drive from filling up with transaction logs, you can remove the affected passive database copy instead of suspending it.

Alternative Solution:
1.  Configured Non-GRT backup of Exchange database, ran it.
2.  After the Non-GRT backup completed ran a GRT backup for Exchange database.
3.  This process truncated logs by first running a Non-GRT backup and then a GRT backup.