Friday 17 April 2009

Exchange 2010 High Availability! Database Availability Groups RIP SCC, CCR and SCR

Rest in peace SCC, CCR and SCR!

Exchange 2010 now only supports one method for High Availability, and that's Database Availability Groups, or DAGs for short.

You can have upto 16 Servers in a DAG, and you aren't limited to just two copies of a database like you were with CCR - you can have 2+ or more copies if you so wish. Also all Mailbox Servers that are Members in a DAG don't necessarily have to have all databases log shipped over to each other.

Benefits of Database Availability Groups!

1. You now fail over at the MAILBOX LEVEL. This means you don't fail over an entire cluster anymore. Your Mailbox Server a little over utilized? Then move 1 or 2 Databases over to a lesser used Mailbox Server to stabilize resources.

2. You can have truly multiple copies of a Database! Technically with 3 or more copies you could use Circular Logging and enter into a Backupless state!

3. DAG Servers can stretch AD Sites! Making DR and HA one solution - without having the pain of having to have an SCR Target.

Play with DAG in the released Exchange 2010 BETA.

Powershell cmdlets:

1. New-DatabaseAvailabilityGroup

This creates the DAG

2. Add-DatabaseAvailabilityGroupMember

This adds a Mailbox Server to the DAG

3. New-DatabaseAvailabilityNetwork

This allows you to specify a Network for Log Shipping

and finally, 4. Get-DatabaseAvailabilityGroup

This shows you all available DAGs, it's members and Operational Servers.

It is noted this is all available to do through the Exchange Management Console too for all those still not quite accustomed to Powershell:

It is worth noting you still need a FileShare Witness if you have an even number of Mailbox Servers in the DAG. You specify this at the time of DAG creation.
DAGs also rely on the use of Failover Clustering, so Windows 2008 Enterprise is still needed.
A very very good feature for smaller companies is that you can have Mailbox Servers as members of a DAG and achieve HA with your Mailbox Data and still also have both the Hub Transport and Client Access Server roles on the Server - effectively total HA with the use of just TWO Servers! This does not take into account Active Directory tin, and also you would need Hardware Load Balancing - as Windows NLB is not supported on a Server with Failover Clustering installed.

That's it for now, I will take you through the steps of creating a DAG and multiple copies of a Database is a future post.

Oliver Moazzezi

MVP - Exchange Server

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