DAG Failover and Journaling is not an area highlighted on TechNet. If you configure journaling on a mailbox database and a failover of that mailbox database occurs what happens with the journaling? Does it still work?
The answer is Yes... Let me explain.
When a journal rule is created it is stored in Active Directory. The journal rule is replicated to all domain controllers throughout your infrastructure and applied to all Hub Transport servers in the Exchange 2010 organization.
When a mailbox database fails over from Site1 to Site2, the hub transport server in Site2 will still have the journal rules applied as it was replicated using Active Directory replication. The store driver on the hub transport servers in Site2 will pull the emails from the failed over database as it would in Site1. The journal agents on the hub transport servers in Site2 would then process the journal rules.
It doesn't matter what location the mailbox database is located, it will always be journaled if the rule is configured!
If you journal all emails coming in and out of a mailbox database the the disk I/O will be two times the typical disk I/O. It is recommended that a separate server be setup to act as a journaling server and to hold the journal recipient mailboxes. If your journal recipient mailbox is located in Site1 and the mailbox database being journaled is located in Site2 due to a DAG failover then all journal emails will be replayed back to Site1 which can cause WAN link congestion.