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Exchange 2010 Database Availability Group (DAG)

Tariq M. Jaber [MCSE 2003, MCTS (ISA 2006, Exchange 2007)]

Tariq M. Jaber [MCSE 2003, MCTS (ISA 2006, Exchange 2007)] Photo

Tariq is a senior Microsoft Systems Engineer. He implemented several Microsoft infrastructure projects for various major companies. He is now focusing on Active Directory and Exchange server administration and implementation.

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In this article we will explain the new Exchange 2010 high availability feature; Database Availability Group (DAG). An overview, how it works and how to deploy a highly available Exchange 2010 environment.

Adding Mailbox Database Copies

Now that we have configured the DAG, we will continue by adding mailbox database copies to start protecting our databases.

We will configure the following scenario:
In all we have two mailbox servers, Ex14Mbx1 and Ex14Mbx2, with two mailbox databases, Main-DB01 and Main-DB02. Ex14Mbx1 holds the active Main-DB01 database copy and a passive copy of Main-DB02, the same applies for Ex14Mbx2; it holds the active Main-DB02 database copy and a passive copy of Main-DB01.

Completed DAG Setup

From EMC | Organization Configuration | Mailbox, click the Database Management tab, and right-click the database for which we want to add a copy

Add Mailbox Database Copy

In the Add Mailbox Database Copy window click browse and select the DAG member that you will configure to host the database copy

Add Mailbox Database Copy - Configuration

In the Add Mailbox Database Copy window, there is an Activation preference number. This value is used when multiple database copies are added for one database and all the copies meet the same criteria for activation. In this case the copy assigned the lowest activation preference number will be activated.

Click add and wait for the command to complete successfully

Add Mailbox Database Copy - Finished

After the copy has been created, we can check the health of the database copy using Exchange Management Console. In Exchange 2007 we had to use the Exchange Management Shell to check for mailbox databases and replication health. Now we can use the Database Management tab and look at the Copy Status colomn

Database Management - Copy Status

Mailbox Database Switchover

The Mailbox server that hosts the active copy of a database is called the mailbox database master. Sometimes you may need to take the mailbox database master down for maintenance. In this case we need to move the active mailbox database to another mailbox server. This process is called a database switchover. In a database switchover, the active copy of a database is dismounted on the master and a passive copy of that database is mounted. The active mailbox database is mounted on another mailbox server which in its turn becomes the master.

To activate the mailbox database on another server, in EMC | Organization Configuration | Mailbox, click the Database Management tab, at the bottom pane right-click the copy that is hosted on the server on which you want to activate the copy

Activate Database Copy

The following drop down list will appear to select from:

Activate Database -  Override Mount

The options in the list are:

  • Lossless If you specify this value, the database doesn't automatically mount until all logs that were generated on the active copy have been copied to the passive copy.

  • Good Availability If you specify this value, the database automatically mounts immediately after a failover if the copy queue length is less than or equal to 6. Exchange will attempt to replicate the remaining logs to the passive copy and then mounts the database. If the copy queue length is greater than 6, the database doesn't mount.

  • Best Effort If you specify this value, the database automatically mounts regardless of the size of the copy queue length. Because the database will mount with any amount of log loss, using this value could result in a large amount of data loss.

  • Best Availability If you specify this value, the database automatically mounts immediately after a failover if the copy queue length is less than or equal to 12. The copy queue length is the number of logs recognized by the passive copy that needs to be replicated. If the copy queue length is more than 12, the database doesn't automatically mount. When the copy queue length is less than or equal to 12, Exchange attempts to replicate the remaining logs to the passive copy and then mounts the database.

Click ok to start activating the copy on the second server. When the process finishes we can see the results in the console:

Activate Database - Complete

We can also activate the mailbox database copy on another server through Exchange Management Shell using the command:
Move-ActiveMailboxDatabase -Identity Main-DB02 -ActivateOnServer Ex14Mbx1

Move-ActiveMailboxDatabase

Conclusion

In this article we went through a brief overview of database availability groups. We introduced DAG, created and configured DAG to include two member servers. We created mailbox database copies within the DAG and tested moving the database copies between member servers.

References

Managing Mailbox Database Copies

Planning for High Availability and Site Resilience

New High Availability and Site Resilience Functionality

Understanding Database Availability Groups

Cluster Continuous Replication

User Comments - Page 1 of 1

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Dinesh Khot 28 May 2014 23:38
Great ... Very Very helpful....



Praba 25 Feb 2014 23:27
Nice explanation for DAG
anonymous 7 Apr 2012 14:11
In the article it is stated that a database copy can be seeded using a non active database copy as the source, if I understood the intent correctly. How is this accomplished because the Add-MailboxDatabaseCopy cmdlet does not show or offer and -source parameter options.
Awesome really helpful 10 Jan 2012 09:27
Awesome really helpful
roger 28 Aug 2011 22:07
How does one protect echange now with out installing a second exchange server.
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