I found a post at Henrik Walther Blog over at MSExchange.org that talk about DAG. http://blogs.msexchange.org/walther/2009/04/18/exchange-2010-database-availability-groups/
It is one of the most interesting new features in Exchange 2010. Here is a list of the new DAG functionality from the blog.
- The new Database Availability Group (DAG) HA/site resilience feature replaces CCR/SCR/LCR
- Also note that SCC has been deprecated/cut with Exchange 2010
- DAG built on the functionality we know from CCR and SCR, that is it still uses asynchronous log shipping and replay etc
- An interesting thing about DAGs is that you’re no longer required to form a cluster before you install the MBX server role
- The limited cluster features that are used by DAGs (primarily cluster heartbeat and quorum) are configured automatically when adding the first MBX server to the DAG and thereby more or less invisible to the administrator
- With DAG you can have up to 16 copies of a Mailbox database
- In addition, you can also have other Exchange 2010 server roles such as HT and CAS installed on the MBX server which is member of a DAG
- Also, you can have DAG members located on different subnets and in separate AD sites
This image over at MSExchange.org describes very well the most important changes in the EMS for administrators regarding DAG.