Some Key Terminology of Exchange 2010

Database availability group (DAG)

A group of up to 16 Exchange 2010 Mailbox servers that hosts a set of replicated databases.

A DAG is the base component of the high availability and site resilience framework built into Exchange 2010.A DAG is a group of up to 16 Mailbox servers that hosts a set of databases and provides automatic database-level recovery from failures that affect individual databases. Any server in a DAG can host a copy of a mailbox database from any other server in the DAG. When a server is added to a DAG, it works with the other servers in the DAG to provide automatic recovery from failures that affect mailbox databases, such as a disk failure or server failure.Exchange 2007 introduced a built-in data replication technology called continuous replication. Continuous replication, which was available in three forms: local, cluster, and standby, significantly reduced the cost of deploying a highly available Exchange infrastructure, and provided a much improved deployment and management experience over previous versions of Exchange. Even with these cost savings and improvements, however, running a highly available Exchange 2007 infrastructure still required much time and expertise because the integration between Exchange and Windows failover clustering wasn't seamless. In addition, customers wanted an easier way to replicate their e-mail data to a remote location, to protect their Exchange environment against site-level disasters.

Exchange 2010 uses the same continuous replication technology found in Exchange 2007. Exchange 2010 combines on-site data replication (CCR) and off-site data replication (SCR) into a single framework called a database availability group (DAG). After servers are added to a DAG, you can add replicated database copies incrementally (up to 16 total), and Exchange 2010 switches between these copies automatically, to maintain availability.

Unlike Exchange 2007, where clustered mailbox servers required dedicated hardware, Mailbox servers in a DAG can host other Exchange roles (Client Access, Hub Transport, and Unified Messaging), providing full redundancy of Exchange services and data with just two servers.

This new high availability architecture also provides simplified recovery from a variety of failures (disk-level, server-level, and datacenter-level), and the architecture can be deployed on a variety of storage types.

For more information about DAGs, see Understanding Database Availability Groups.

Database mobility

The ability of a single Exchange 2010 mailbox database to be replicated to and mounted on other Exchange 2010 Mailbox servers.

Disaster recovery

Any process used to manually recover from a failure. This can be a failure that affects a single item, or it can be a failure that affects an entire physical location.

High availability

A solution that provides service availability, data availability, and automatic recovery from failures that affect the service or data (such as a network, storage, or server failure).

With the significant core improvements made to Exchange 2010, the recommended maximum mailbox database size when using continuous replication has increased from 200 gigabytes (GB) in Exchange 2007 to 2 terabytes in Exchange 2010. With more companies realizing the greater value in large mailboxes (from 2 GB through 10 GB), significantly larger database sizes can quickly become a reality. Supporting larger databases means moving away from legacy recovery mechanisms, such as backup and restore, and moving to newer, faster forms of protection, such as data replication and server redundancy. Ultimately, the size of your mailbox databases depends on many factors you derive during the Exchange 2010 planning process for. For detailed planning guidance for mailboxes and Mailbox servers, see Mailbox Server Storage Design.

Lagged mailbox database copy

A passive mailbox database copy that has a log replay lag time greater than zero.

Mailbox database copy

A mailbox database (.edb file and logs), which is either active or passive.

Mailbox resiliency

The name of a unified high availability and site resilience solution in Exchange 2010.

Site resilience

A manual disaster recovery process used to activate an alternate or standby datacenter when the primary datacenter is no longer able to provide a sufficient level of service to meet the needs of the organization. Also includes the process of re-activating a primary datacenter that has been recovered, restored or recreated. You can configure your messaging solution for high availability and enable site resilience using the built-in features and functionality in Exchange 2010.