Hi team,

If there are two datacenter sites and I need to enable Active/Active with Multiple DAG implementation, can someone please explain me if the following Mailbox Server Placement is correct?

I need to know if it's possible to host the Two databases within the Two DAGs in the 4 Servers with CAS collocated as shown above.

Why I'm concerned on this is, when I use the Exchange 2013 Calculator and the below inputs, the Role requirement page gives a total of 8 servers for Mailbox role. I thought I can achieve this by using 4 total across both datacenters(2 + 2 Servers hosting the 2 Database Availability Groups pretty much as shown in the above image) What am I missing here.

Thank You,
Chris!

Hi Chris

A mailbox server cannot be a member of 2 different DAGs so the above design will not work.  If you wanted to only use 4 servers then you would have 2 servers per DAG in an Active/Active design.

Steve

Hi Steve,

Thank you for the clarification. So in this scenario, if I add additional two Mailbox Server members and configure the DAG02 on that with DB2 I guess that solution falls into the working scenario?

Also, since I'm looking for HA within the Datacenter as well with Active/Active sites, what if I add both DB1 and DB2 to DAG01 and proceed without a DAG two? Here I'm not using the Multiple DAGs, but can I achieve Active/Active within the Datacenter and across Datacenters?

What option do you think would be the best? :)

Cheers!
Chris

The thing that you are protecting yourself from with 2 DAGs is link failure.  In the diagram above if the link went down the databases in B (assuming that is your secondary site) would dismount.  If you had 2 DAGs then A would be primary for DAG1 and B would be primary for DAG2 and the primary databases in DAG would stay mounted in the event of a link failure.

Now this may not be an issue if a) your link is resilient or b) you can live with downtime in Datacenter B.

Outside of that you can have active databases in both sites in a single DAG as long as the link is up.

Steve

Thank you Steve,

Can you just check if the sizing i did here is correct, since from the description of the calculator for "Database Availability Group Multiplier" says "Enter the number of database availability groups you will deploy in your environment." and I'm a bit confused in what to put as the value, 1 or 2. Since if I add 1 the servers go as below, 

And if I add 2 (since the description says "Enter the number of DAGs you will deploy in your environment..) the number of Total Mailbox servers per environment increases to 16. Am confused on this part since I'm looking to have Active/Active within 2 Datacenters for 2 DAGs using 8 servers total.

Thank You.
Chris!

Hi Chris

I guess that is just a quirk with the language in the calculator.  If you look at the /Environment column you can see that it lists there the number of DAGs as "2" and in the /DAG column you have Number of Mailbox Servers / DAG as "4" so the total here is correctly displayed as 8.

Steve