Database Availability Group In Exchange Server 2010

One of the nice feature which I like most in Exchange Server 2010 is the new Database Availability Group (DAG) high availability and site resilience feature which has replaced LCR/SCR/CCR.

Cluster is no longer required before installing the MBX server role in DAG. Cluster heartbeat and quorum are configured by-default to the DAG when adding first Mailbox Server and hence more or less visible to the administrator.

A DAG is a group of 16 Mailbox servers that can each maintain up to 100 databases (up to 1600 databases in a DAG) and each database can have 16 copies of a Mailbox database with DAG using continuous replication. In addition, we can also have other Exchange 2010 servers such as Hub Transport and CAS which can be the member of a DAG. DAG members can be on different subnets or on different Active Directory sites.

As we know, Exchange Server 2007 introduced a number of new options for availability, which includes Cluster Continuous Replication (CCR), Single Copy Cluster (SCC), Standby Continuous Replication (SCR) and Local Continuous Replication (LCR). You can read more about them from this link.

How the DAG differs from Exchange Server 2007 SP1 availability options, let’s see:-

1.       With CCR, we can have only two highly available copies of the database within the cluster, but DAG there can be up to 16 copies of each database.

2.       SCR required manual administrative intervention for activation, whereas DAG provides automatic database failover.

Therefore we can conclude that Exchange Server 2010 provides database-level failover within the DAG which means failure of single database no longer affects all mailbox databases on a server. Also, DAG handles both in-site and inter-site replication for implementing site failover.  

 A DAG is initially empty, and a directory object is automatically created in Active Directory that represents the DAG. This object is used to store related information about the DAG, such as server membership. A failover cluster is automatically created when an administrator adds the first server to a DAG. The failover cluster heartbeat mechanism and cluster database are then used to track and manage information about the DAG that can change quickly, such as database mount status, replication status, and last mounted location.

What has been removed?

1. No more Exchange Virtual Server/Cluster Mailbox Server
2. Database is no longer associated to a Server but is an Org Level resource
3. There is no longer a requirement to choose Cluster or Non Cluster at installation, an Exchange 2010 server can move in and out of a DAG as needed
4. The limitation of only hosting the mailbox role on a clustered Exchange server
5. Storage Groups have been removed from Exchange

Continuous replication creates a passive database copy on another mailbox server in the DAG and uses asynchronous log shipping to maintain the copies. Unlike in Exchange Server 2007 which uses SMB (file share) protocol for transactional log shipping, Exchange Server 2010 uses TCP sockets default port 64327 for replication. Current used port can be checked by running the Get-DatabaseAvailabilityGroup -Status | Format-List command.

Block mode which was introduced in the Exchange Server 2010 SP1- which reduces the exposure of data loss during failover by replicating all logs to the passive database copies in parallel to writing them locally. In other words, it reduces the possibility of any data loss during a failover and the time it takes to perform a switchover.