How to understand the concept of Exchange 2010 database replicas

The key characteristics of a mailbox database replica are:

1 The database copy applies only to the mailbox database. To achieve redundancy and high availability of public folder databases, it is recommended that you use public folder replication.

2 You can create up to 16 copies of the Exchange 2010 mailbox database on multiple mailbox servers, as long as the servers are grouped into the database availability Group (DAG), which is the boundary of continuous replication.

3 Exchange 2010 Mailbox databases can be replicated only to other Exchange 2010 mailbox servers in DAG. You cannot replicate a database DAG external, nor can you copy an Exchange 2010 mailbox database to a server that is running Exchange 2007.

4 All mailbox servers in DAG must be in the same Active Directory domain.

5) As with standby continuous replication (SCR), all mailbox database replicas support the concept of replay latency and truncation delay time. However, before you enable these features, you must plan carefully.

6 Backup all copies of the database using the aware Exchange and backup replication based on Volume Shadow Copy Service (VSS). However, built-in support for Windows server backups is available only for active replicas. You cannot use a Windows server backup to back up a passive copy.

7 A copy of the database can only be created on a mailbox server that is active (mounted and in use) of an unmanaged database. You cannot create two copies of the same database on the same server.

8 All replicas of the database use the same path on each server that contains replicas. The database and log file paths for a copy of the database on each mailbox server must not conflict with any other database path.

9 You can create a copy of the database in the same or different Active Directory sites and create a copy of the database on the same or different network subnets.

10 a database copy is not supported between mailbox servers with a roundtrip network delay greater than 250 milliseconds (ms).

Mailbox Database Copy

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You can create a copy of the mailbox database at any time. A copy of a mailbox database can be distributed to a mailbox server in a flexible and granular manner. You can replicate one, some, or all mailbox databases on a server in a variety of ways.

You can create a copy of a mailbox database by using the Add Mailbox Database Copy Wizard in the Exchange Management Console or by using the Add-mailboxdatabasecopy cmdlet in the Exchange command shell.

When you create a copy of a mailbox database, you specify the following information:

1 The name of the database to copy. The database name must be unique within the Exchange organization.

2 The name of the mailbox server that will host the copy of the database. This server must be a member of the same DAG and is forced to host a copy of the database.

3 The amount of time (in minutes) that the log replay is delayed. This time is called the replay delay time. If the value of the replay delay time is set to 0, the log replay delay is turned off.

4 The amount of time (in minutes) that the log truncation is delayed. This time is referred to as "truncation delay time." If the value of the truncation delay time is set to 0, the log truncation delay is turned off.

5 "Activate preferences" number. This is a numeric value that can be used to disconnect a relationship during database activation when multiple copies of the database meet the same activation condition. This value is viewed when the activity manager determines that more than one copy of the database satisfies the same activation condition. A copy with the lowest activation preference number assigned will be activated.