REF: REF: Understanding Exchange 2010 DAG – Part 1

 

1. Database Availability Groups (DAGs)

Exchange 2010 does not use the concept of a clustered mailbox server (referred to as an Exchange Virtual Server in Exchange Server 2003 and earlier). As a result, Exchange is no longer a clustered application, and Exchange server identities no longer move between clustered servers.

Exchange 2010 does not use the concept of storage groups. As a result, databases are decoupled from servers and are now managed globally, databases no longer share log streams, and continuous replication (including switchovers and failovers) operates at the database level.

Exchange 2010 does not use the concepts of public and private networks. These concepts are replaced with the concepts of MAPI networks and Replication networks. Each DAG should contain one MAPI network and one or more Replication networks.
(No More Storage Group/ )

2. Creating one or more DAGs

• The DAG is configured for Third-Party Replication mode, and you want to revert to using continuous replication. You cannot convert a DAG back to continuous replication; you need to create a new DAG.
• You have servers in multiple domains. All members of the same DAG must also be members of the same domain.

3. Configuring DAG properties

• Witness Server and Witness Directory
  The Witness Server is a server outside the DAG that acts as a quorum voter when the DAG contains an even number of members. The Witness Directory is a directory created and shared on the Witness Server for use by system in maintaining quorum.
  (Witness Server/ Quorum voter)
  
  
• IP Addresses
  Each DAG will have one or more IPv4 addresses, and optionally, one or more IPv6 addresses. The IP addresses assigned to the DAG are used by the DAG's underlying cluster. The number of IPv4 addresses assigned to the DAG equals the number of subnets that comprise the MAPI network used by the DAG. You can configure the DAG to use static IP addresses or to obtain addresses automatically by using Dynamic Host Configuration Protocol (DHCP).
  (Static IP/ DHCP/ MAPI network)
  
  
• Database Activation Coordination (DAC) Mode
  DAC mode is a property setting on a DAG that is designed for DAGs with three or more members that have been deployed to multiple sites. DAC mode is used to deal with conditions that would otherwise lead to a split brain syndrome within the DAG, such as a site failure. For more information about DAC mode, seeUnderstanding Datacenter Activation Coordination Mode.
  (DAC Mode/ Mulitple Site/ Site Failure)
  
  
• Alternate Witness Server and Alternate Witness Directory
  The Alternate Witness Server and Alternate Witness Directory are values that you can pre-configure as part of the planning process for DAGs that are configured for site resilience.
  (Alternate Witness Server/ Alternate Witness Directory)
  
  
• Replication Port
  By default, all DAGs use TCP port 64327 for continuous replication. You can modify the DAG to use a different TCP port for replication by using the ReplicationPortparameter of the Set-DatabaseAvailabilityGroup cmdlet.
  (PS>: Set-DatabaseAvailabilityGroup/ ReplicationPortparameter)
  
  
• Network Discovery
  You can force the DAG to rediscover networks and network interfaces. This operation is used when you add or remove networks or change DAG network subnets. Rediscovery of all DAG networks can be forced by using the DiscoverNetworks parameter of the Set-DatabaseAvailabilityGroup cmdlet.
  (DiscoverNetworks)
  
  
• Network Compression
  By default, DAGs use compression only between DAG networks that are on the same subnet. You can enable compression for all DAG networks or for seeding operations only, or you can disable compression for all DAG networks.
  (compression/ DAG networks/ same subnet)
  
  
• Network Encryption
  By default, DAGs use encryption only between DAG networks that are on the same subnet. You can enable encryption for all DAG networks or for seeding operations only, or you can disable encryption for all DAG networks.
  (DAG networks/ enable or disable encryption/ same subnet)
  

4. DAG Networks

Each DAG member must have at least two network interface cards (NICs). One NIC is used for the MAPI network, and one NIC is used for the Replication network. Additional NICs can be added to create additional Replication networks, for use as dedicated backup networks, or for use by the system as iSCSI storage. DAG network management involves designating a network as a MAPI network or as a Replication network, and configuring network subnets.
(Two NICs/ MAPI network/ Replication Network and More NICs)

5. Shutting down DAG members
The Exchange 2010 high availability solution is integrated with the Windows shutdown process. If an administrator or application initiates a shutdown of a Windows server in a DAG that has a mounted database which is replicated to one or more other DAG members, then the system will try to activate another copy of the mounted database(s) prior to allowing the shutdown process to complete. However, this new behavior does not guarantee that all of the databases on the server being shut down will experience a loss-less activation. As a result, it is a best practice to perform a server switchover prior to shutting down a server that is a member of a DAG.
(Windows Shutdown Process/ SwitchOver)

 

Blog Extended Reading

More Information & Reference

1. Managing High Availability and Site Resilience