Wednesday, 3 March 2010

Redundency of Exchange 2010 Mailbox Database Servers

In an earlier post, I covered the creation of a DAG ( Database Availability Group) to provide mailbox database redundency. How does this differ to earlier methods of redundency used by Exchange 2007? Both CCR (Continuous Cluster Replication) used in Exchange 2007 and DAG used in Exchange 2010 are offshoots of Windows Failover Clustering (WFC).

CCR – as its name implies – works by allowing you to create a cluster during the installation of Exchange 2007.  This one is a bit easier to see as part of WFC, as you have to create a Failover Cluster first – specifically a Distributed Majority-Node File Share Witness Failover Cluster.  After that, when you install Exchange Server you can specify which server(s) will be the Active node(s) and which will be passive.  This creates the clustered Exchange resources for you, making the overall process of setting up clustering for Exchange a lot easier.

DAG will permit you to create the cluster itself from Exchange 2010 command sets, eliminating the need to pre-create the Failover Cluster prior to getting the Exchange installation rolling.  While this makes the process even easier than in 2007, it still requires that you have two or more servers capable of running Distributed Failover Clustering.  This means that not every version of Windows 2008 is going to be suitable for DAG, but also means that – under the hood – you still need to know how Distributed Failover Clustering works to properly manage the DAG systems.

How Does Exchange Determine How Databases Failover?

When the active database fails, failover to a replicated copy occurs automatically within a time period of aound 30 seconds. But which passive copy is chosen? The component that determines this is the Active Manager.

The Active Manager has essentially two parts , the SAM and PAM:
  1. The Primary Active Manager which sits on just one of the DAG mailbox servers. This determines which replica should be active.
  2. The Stanby Active Manager which sits on every other mailbox server in the DAG. This notifies the PAM of when a database fails and also the HTs  and CAS servers about which replica is active.
When the active copy fails the Active Manager makes a determination of which replica should be targeted to become active. This involves looking at the database copy status, health of the content index and length of the log copy queue. After determining potential targets, the database replica with the lowest Activation Preference Number will win.

To set this value on a database replica type the following:

[PS] Get-MailboxDatabase  -Server srv211 | fl name,act*

This cmdlet will show you the current activation preference numbers for the database replica (SRV211 hosts a replica database).

[PS] Set-MailboxDatabaseCopy -Identity DB01\SRV211 -ActivationPreference 2

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