Exchange Server 2010 : Utilize the Availability Options for Servers Based on Role (part 2) - Increase Mailbox Database Availability

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2. Increase Mailbox Database Availability

One of the primary areas of focus for keeping a high degree of availability is the data that each user stores in their mailbox. Maximizing mailbox database availability is one of the most difficult challenges in managing Exchange. The capabilities around database availability have changed from one Exchange version to the next, with each method becoming increasingly more effective. This section helps you understand how to maintain high levels of availability for your mailbox databases in Exchange Server 2010.

2.1. Understand Database Availability Options

The options for database availability have changed in Exchange Server 2010. In previous versions of Exchange, you had the ability to cluster mailbox servers in order to keep mailbox databases highly available. In Exchange Server 2007, continuous replication was introduced, which provided database availability by copying the transaction logs from an active database to a passive copy of the same database.

Exchange 2010 further expands on the continuous replication technology and provides a single database availability solution that fits every common scenario. This technology is referred to as a database availability group (DAG).

2.1.1. Understand the Basics of DAGs

A DAG is a collection of Mailbox servers (up to 16 servers) that can share copies of the same mailbox databases. One server in the DAG holds the active copy of the database, and one or more other servers can contain passive copies of the same database. The databases are kept synchronized using the continuous replication log shipping method that was used in Exchange Server 2007. There are many advantages to the DAG approach:

  • Databases can be hosted on any of the Mailbox servers in the DAG.

  • Mailbox servers in a DAG can also perform other Exchange roles.

  • Recovery from a database failure is automatic.

  • Mailbox servers in a DAG can live in different sites.

  • DAGs use Windows Failover Clustering in the back end, but all configuration and administration is performed through the Exchange tools.

  • Failover occurs within 30 seconds.

Figure 6 shows how a DAG configuration might look.

Figure 6. An example of a DAG configuration

In this example, there are three Mailbox servers and five databases that are replicated between the mailbox servers in the DAG. Only one server has an active copy of the database. As you can see in the figure, the database can be selectively replicated to other servers. Each server is not required to host a replica of every database. This offers great flexibility in designing your database availability solution.

2.1.2. Determine How Databases Fail Over

When the active copy of a replicated database fails, how does Exchange determine which passive copy gets activated? This process is determined by a component called the Active Manager. The Active Manager has two roles:

  • The Primary Active Manager (PAM) is hosted on one of the servers in the DAG. The PAM determines which copy of the databases should be active.

  • The Standby Active Manager (SAM) is hosted on the remaining Mailbox servers in the DAG. The SAM keeps tabs on the databases running on the Mailbox server and notifies the PAM when a database fails. The SAM also informs Hub Transport servers and Client Access servers about which server hosts the active copy of a database.

When a database failure occurs, the Active Manager looks at the health of the passive copies of the databases to determine which copies should be targeted for failover. In making its determination, the Active Manager looks at the database copy status, the health of the content index, and the length of the log copy queue and the log replay queue.

If multiple passive copies of the database meet the Active Manager's criteria list, the database with the lowest Activation Preference setting is activated. The activation preference order is determined by you when you add database copies to other Mailbox servers in a DAG.

2.2. Implement Database Availability Groups

To implement DAGs, use the following steps:

  1. Configure the network adapters on the mailbox servers that will be in the DAG.

    Every mailbox server in the DAG must have two adapters installed. One adapter will be for clients to access the data in the mailbox stores (MAPI traffic), and the other adapter will be used for data replication between DAG members. These two adapters must be on different subnets and the subnets must map to networks that are assigned to the DAG.

  2. Create the DAG.

  3. Add members to the DAG. Up to 16 members can be added. When a member is in the DAG, it can host copies of databases from any other member in the DAG.

  4. Create some mailbox databases. If you don't already have a mailbox database on one of your Mailbox servers in the DAG, go ahead and create some.

  5. Add a copy of a mailbox database in a DAG to another server in the DAG. Now that you have multiple servers in your DAG and at least one database, you can enable that database to be copied to other servers.

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  •  Programming WCF Services : Queued Services - Instance Management
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  •  Exchange Server 2010 : Implementing Client Access and Hub Transport Servers - Test Cmdlets for CAS and Hub Transport Servers
  •  Exchange Server 2010 : Implementing Client Access and Hub Transport Servers - Installing the Hub Transport Server
  •  Exchange Server 2010 : Implementing Client Access and Hub Transport Servers - Transport Pipeline
  •  Exchange Server 2010 : Implementing Client Access and Hub Transport Servers - Understanding the Hub Transport Server
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