Database Availability Group (DAG) is the new high availability solution in Exchange 2010. None of the clustering technologies in 2007 is valid in 2010.
DAG is a collection of upto 16 mailbox servers with a maximum of 16 copies of each database. It makes the database independent of any server and gives us failover on a mailbox level rather than a hardware or storage group level as in 2007. To illustrate it, if one database gets corrupted or the disk having the files fails, you can quickly mount a copy of the same database in any of the servers which is part of the DAG.
Follow the steps to create and configure a DAG.
Launch EMC 2010 and navigate to Organization Configuration –> Mailbox –> Database Availability tab. Right click anywhere on the pane and select “New Database Availability Group”. You can also select the option from the action pane.
You need to specify the name of the DAG (this will appear in EMC), the file share witness unc path (similar to 2007) and the location of the FSW folder. Exchange will create the folder and hence specify the path as to where you want the folder to be. Microsoft recommends to have the FSW on a hub server.
Click New and Finish. You will see that the DAG is now listed in EMC.
The creation of DAG will only create an AD computer object, which is visible in AD Users and Computers.
Just having a DAG created is no good to us. Now, we will add the first node to the DAG. Right click the DAG in EMC and click on “Manage DAG Membership”.
Select the first server to be part of the DAG and click Manage.
Using the console for adding the first server gave me the error below stating that “Insufficient permissions to view the network shares on the machine, Error Code 5”.
The problem here is that remote powershell is used in the background which is causing the error. It is a known issue and should be corrected in the final release. I launched the Local Powershell and ran the same command which the console ran and everything was fine.
Add-DatabaseAvailabilityGroupServer –identity HEWDAG –MailboxServer E14Node1
As you can see, the shell installs Windows Failover Clustering as part of configuring the server to be a member of DAG. You don’t have to install clustering before hand as the process will go ahead and install if it is not present. This is good for admins who are not that comfortable with configuring the cluster themselves, like CCR in 2007.
Once the server becomes part of the DAG, more information is made available in the console.
The failover clustering quorum settings will be set to “Node Majority”.
In order to add the second server E14Node2 to the DAG, I launched the Local Powershell from the second server and executed the same command which we ran earlier.
Add-DatabaseAvailabilityGroupServer –identity HEWDAG –MailboxServer E14Node2
Once the second node is added, the quorum setting is automatically changed to “Node and file share majority”.
The file share witness is only created once the second server is added to the DAG as the quorum setting is only changed at that time.
The EMC will now show both the servers as part of the DAG in the EMC.
That is it. We now have a working DAG. I will explain the database copy and failover functionality in a different article.
Thanks Deepak.
Deepak Khandelwal:
http://www.thecabal.org/2009/12/busting-the-exchange-trusted-subsystem-myth/
Hi Rajith
In Exchange 2010 RC, some of the steps and screenshots have changed but process is mostly same. It is made more easy now and you do not have to create the witness share and folder beforehand, wizard will do it for you when you add member in the group.
Note: for Witness server if you use any other server other than Exchange 2010 you will get this error when you create a witness
The following warning(s) occurred while saving changes:
Set-DatabaseAvailabilityGroup
Completed
Warning:
Specified witness server DTDC1.DKTEST.LOCAL is not an Exchange server, or part of the Exchange Servers security group.
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Warning:
Insufficient permissions to access file shares on witness server 'DTDC1.DKTEST.LOCAL'. Until this problem is corrected, the database availability group may be more vulnerable to failures. You can use the Set-DatabaseAvailabilityGroup cmdlet to try the operation again. Error: Access is denied
Warning:
The command completed successfully but no settings of 'DAGGroup1' have been modified.
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To resolve the error add the witness server to member of Security group "Exchange trusted subsystem" and then add the "Exchange trusted subsystem" as member of local administrators group in witness server.
Thanks Sam.
Very good article.