Exchange 2010 DAG's - 4 Mailbox Servers in 2 Datacenters with per site redundancy

Question

I recently came into a company with an Exchange 2010 sp1 environment where there are two data centers in two different subnets (10.70.62.0 and 10.80.56.0)
Each datacenter has 2 mailbox servers (4 servers total) but their plan was to have only enough storage space to host 1/2 of the databases on each server. So instead of server 1, 2, and 3 being copies of each other, ServerA is a copy of ServerA and ServerB
is a copy of ServerB
Server     -            Databases
Site1mbA - Primary Database copy 1-10
Site1mbB - Primary Database copy 11-20
Site2mbA - Secondary Database copy 1-10
Site2mbB - Secondary Database copy 11-20
Normally if there were enough storage space I would keep a copy on each server and replicate within 1 DAG.
However since not all the databases are on each member server do I need to use 2 DAGS? 1st for Site1mbA & Site2mbA and 2nd for Site1mbB & Site2mbB
I feel I would need the 2 DAG's because with 1 DAG if Site1mbA server went offline the DAG couldn't switch to the Failover Cluster IP for Site2 because there are still active mailboxes in Site1 on Site1mbB.
Does that make any sense or am I over complicating it?
Thanks!

Answer 1

There is no rule that every server in a DAG must have a copy of every Database. You could do this with one DAG, but if the WAN goes down then the DAG will be active in only a single site not both.
http://technet.microsoft.com/en-us/library/dd979781(v=exchg.141).aspx#FourTwo
As illustrated in the previous example, using a single four-member DAG extended across two datacenters can provide high availability and site resilience for the mailbox services and data. However, if a WAN outage occurs, only the primary datacenter retains
service because it contains the majority of the voters. The datacenter with the minority of voters loses majority, and the DAG members in that datacenter lose quorum and go offline.
To deploy highly available Mailbox servers in a multiple datacenter environment, where each datacenter is actively serving a local user population, we recommend that you deploy multiple DAGs, where each DAG has a majority of voters in a different datacenter,
as illustrated in the following figure.
Cheers,
Rhoderick
Microsoft Senior Exchange PFE
Blog:
http://blogs.technet.com/rmilne 
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