How To Split Exchange Server DAG

Database Availability Group (DAG) setup in Exchange Server provides full protection and recovery in case of database, network, or server failure. In a DAG, you can have up to 16 mailbox servers hosting a set of databases. This helps provide automatic, database-level recovery in case something happens. 

Sometimes, situations arise when you need to split the Database Availability Group (DAG) and move the mailboxes from one availability group to another. Some common situations are:

• There is a need to segregate the data due to legal obligations or compliance. 
• The data cannot rest with other mailboxes due to privacy or sensitivity of data. 
• A part of the company or a sister company needs to be merged or split. So, you need to also separate the servers. 
• There is a huge amount of data on the servers, and you need to split the server’s resources or split the server to reduce the load on the Database Availability Group (DAG). 

Due to an issue or extreme load, both the active copy and the passive copy are acting as the active copy, causing a huge issue.

Below, we will see how to split the Database Availability Group (DAG) in the Exchange Server.

Process of Splitting the DAG in Exchange Server

The process of splitting Exchange Server DAG involves adding the databases as active copies on the other servers, removing the servers from the current group, and then adding the servers to the new DAG. However, it is easier said than done, as there are various things that could go wrong.

So, first, you need to pause the backup on the specific databases to ensure the full performance of the services during the procedure.

Let’s take an example (see the image below). You need to split the DAG into two groups where Server 1 and Server 2 form part of a group and other servers. The databases need to be split, having Database 1 and 2 with Server 1 and Server 2. On the other hand, databases 3 and 4 would be with the group of Server 3 and Server 4.

The first step is to set the active copy of databases 3 and 4 on servers 3 and 4. After this process is complete, you need to remove the passive copies from servers 1 and 2.

To activate the passive copies, open the Exchange Admin Center (EAC) and go to Servers > Databases. In the details pane, under Database Copies, click Activate. 

Alternatively, you can use the Move-ActiveMailboxDatabase command (as given below) in the Exchange Management Shell (EMS).

Move-ActiveMailboxDatabase <database name> -ActivateOnServer <server to activate database on> -MountDialOverride:None 

Once this is complete, you need to remove the servers that you don’t need in the group. To remove the servers,

• Open the Exchange Admin Center (EAC) and click on Servers > Database Availability Groups. 
• Select the DAG and use the minus (-) button to remove the servers from the group. 
• Then, click Save and Close.

After this, you can create a new Database Availability Group (DAG), add servers 3 and 4 to the group, and then initiate the replication. 

To Conclude

There are various reasons for splitting the Database Availability Group (DAG) in the Exchange Server. If you need to split the DAG, you can follow the process mentioned above. However, you need to ensure that all is working well and there were no issues during the transfer of roles and services. If any issue arises, then it can impact the functionality of the Exchange Servers or damage the databases. If there are any issues, you can easily rebuild the servers. However, the difficult part is recovering the data. 

For easy recovery of data from damaged or corrupted databases, you can take the help of third-party tools like Stellar Repair for Exchange. This tool can repair corrupted databases, with no size limit from any version of Exchange Server, and recover all the data. It allows the granular export of the recovered data to PST and other file formats, such as EML, HTML, PDF, and MSG. The tool can also export the recovered data directly to a live Exchange Server of any version. It can process user mailboxes, user archives, disabled mailboxes, shared mailboxes, and even public folders.