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Get to know about an advanced Exchange Server feature – Database Availability Groups or DAG – for multiple server storage, replication, and failover mechanism.
Database Availability Group or DAG is usually a set of 16 mailbox servers (of the same version) with databases hosted in them. It is the component of the Exchange Server’s mailbox server high availability and site resilience framework. It acts as a boundary for Active Manager (internal component for individual servers to manage switchovers and failovers in DAG), mailbox database replication, database/servers failovers/switchovers.
DAG offers database-level recovery from issues and failures such as disk failures, network problems, server crashes or failures, etc., that badly impact the servers and databases. A DAG server hosts the copy of a mailbox database of another server and works together with added servers to deliver an automatic database-level recovery after the aforementioned failures.
Note: Exchange administrators can create DAG with physical and virtual mailbox servers given that these servers along with solutions fulfill Exchange Server system and Exchange Server virtualization requirements
Setting Up a Database Availability Group (DAG)
Once the Exchange Server is installed, administrators can create DAG, then add mailbox servers to it and further replicate mailbox databases between DAG members.
A DAG is created as an empty object in the Active Directory to store information about server membership and configuration settings. On the addition of a server to the DAG initially, a failover cluster is created automatically, which is solely dedicated to DAG use. DAG can be created with or without a cluster administrative access point (on Exchange 2019, Exchange 2016, Exchange 2013, Windows Server 2012 R2 Standard/Datacenter addition only). When DAG is created without a cluster administrative access point, cluster object name not created in AD, DAG name is not registered in DNS; IP address is not assigned in DAG, and no network name assigned to the cluster.
Before creating the DAG, the user or administrator must have high availability and site resilience permissions and have specified witness server and directory for an even number of Exchange nodes. This witness server contains a folder accessible to the Exchange Trusted Subsystem AD Universal group and is used to maintain the quorum.
The Exchange administrator needs to add the Exchange Trusted Subsystem AD group to the local Administrators group and create a new folder for it with a relevant name for witness server configuration.
Creating DAG via Exchange Admin Center
- Log in to your Exchange Admin Center with administrator credentials.
- Navigate to servers>database availability groups.
- Click the plus icon to create a new group.
- The new database availability group page will be opened. Here, provide the following information details.
- Database availability group name – Unique name with up to 15 characters used as same cluster name
- Witness server – Hostname/Fully Qualified Domain Name (FQDN)
- Witness directory – Directory path on the witness server to store witness data (a default path will be created if this space is left blank)
- Database availability group IP addresses – Static IPv4 addresses to DAG; can be more than one, and space can be left blank to let DAG obtain IPV4 addresses using DHCP
- After entering all the above details, click the Save
- A DAG will be created successfully. Confirm its availability via navigating to servers>database availability groups
Creating DAG via Exchange Management Shell
- Run the Exchange Management Shell application as an administrator.
- Execute the following command to create a DAG to use a witness server and a witness directory.
New-DatabaseAvailabilityGroup -Name <DAG NAME> -WitnessServer <SERVER NAME> -WitnessDirectory C:\<DAG NAME>
- Run this command to check for the successful creation of the DAG.
Get-DatabaseAvailabilityGroup <DAGName> | Format-List
Monitoring Database Availability Group
Exchange Management Shell includes a cmdlet, namely Get-MailboxDatabaseCopyStatus to monitor mailbox database copies for DAGs. It outputs the mailbox database copy status like Failed, Seeding, SeedingSource, Suspended, Healthy, ServiceDown, Initializing, Resynchronizing, Mounted, Dismounted, Mounting, Dismounting, DisconnectedAndHealthy, DisconnectedAndResynchronizing, FailedAndSuspended, and SinglePageRestore.
To check the status of all database copies on the local mailbox server, here is the command.
To check the status of all database copies on the specific mailbox server, run this command.
The unfortunate thing is that Exchange database corruption can occur in spite of all these features. In such situations, one needs to use ESEUTIL for Exchange Server repair. Also, one can try a professional Exchange recovery solution – Kernel for Exchange Server.
This tool is developed to efficiently recover Exchange Server mailboxes, public folders, and archive/shared mailboxes to save them to live Exchange Server, PST, Office 365, and more destinations.
Database Availability Groups keep mailbox database copies on multiple servers and reduces data loss chances. It offers database-level recovery in cases of failures. The basic DAG concept and methods to create DAG are explained to help Exchange Server users.