DAC Mode – Exchange 2010 RTM Vs SP1

Exchange 2010 RTM includes a configuration mode for DAG site resilience support called Datacenter Activation Coordination (DAC) mode. In DAC mode, Exchange cmdlets can be used to perform a datacenter switchover. In the RTM version, DAC mode is limited to DAGs with at least three members that have at least two or more members in the primary datacenter.

In Exchange 2010 SP1, DAC mode has been extended to support two-member DAGs that have each member in a separate datacenter. DAC mode support for two-member DAGs leverages the witness server to provide additional arbitration. In addition, DAC mode has been extended to support DAGs that have all members deployed in a single Active Directory site, including single Active Directory (AD) sites that have been extended to multiple locations.

The Exchange 2010 RTM DACP protocol blocked DAC mode use for DAGs that have less than three nodes. This is viewed as a take back from Exchange 2007, where Cluster Continuous Replication (CCR) looks like a two node DAG, and it is possible to have a two-node datacenter resilient configuration. This two node DAG may be in a single AD site stretched across datacenters, or as it is possible in Exchange 2010, may be stretched across AD sites.

Scenarios :

The following scenarios describe typical situations where Exchange 2010 SP1 DAC mode support can be implemented to provide site resiliency.

Scenario 1: Site resilience for a four node DAG during power failure

The customer has deployed a four node Exchange 2010 RTM DAG between two datacenters in two different AD sites. The customer is very happy with the overall solution. To provide site resiliency, the customer configures DAC mode for the DAG topology.

When a complete power failure happens in the primary datacenter the administrator initiates recovery using the DAC mode tasks. The secondary datacenter is activated. When the power failure is corrected for the primary datacenter it comes online but without communication to the activated standby datacenter. In spite of the recovery the system detects the broader failure and does not mount databases in the primary datacenter.

Scenario 2:  Site resilience for a two node DAG across 2 AD sites

The customer has deployed a two node Exchange 2010 RTM DAG (which may include all roles) between two datacenters in two different AD sites. The customer is very happy with the overall solution until he finds out that DAC mode is not supported for a two node DAG.  He has upgraded from a CCR topology and expected improved site resilience. The customer discovers that Exchange 2010 SP1 does not have this limitation and after updating the member servers, configures DAC mode for the DAG topology.

Scenario 3:  Site resilience for a two node DAG in a single AD site

The customer has deployed a two node Exchange 2010 RTM DAG (which may include all roles) between two datacenters in a single AD site. The customer is very happy with the overall solution until he finds out that DAC mode has a two or greater AD site limitation. He has upgraded from a stretched CCR topology and expected improved site resilience.  The customer discovers that Exchange 2010 SP1 does not have this limitation and after updating the member servers, configures DAC mode for the DAG topology.

Exchange 2010 SP1 introduces enhancements to DAC mode support to provide the following feature changes:

  • Removes the two node limitation for DAC mode support
  • Removes the two AD site limitation for DAC mode support
  • Provide a solution that is easy to implement, safe and not prone to error without sacrificing the reliability provided by Exchange 2010 RTM DAC mode
Advertisements
This entry was posted in Exchange Servers. Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s