We all know that in the Microsoft Exchange world DAG stands for “Database Availability Group”.
Database – because on a highly available Exchange 2010 Mailbox server, the database, not the server, is the unit of availability and it is the database that can be failed over or switched over between multiple servers within a DAG. This concept is known as database mobility.
Group – because the scope of availability is determined by Mailbox servers in a DAG combined in a failover cluster and working together as a group.
Availability – this word seems to be the least obvious and the most obfuscated term here (and also referred to by both other terms). Ironically, it has a straightforward mathematical definition and plays an important role in understanding overall Exchange design principles.
Wikipedia defines “availability” to mean one of the following:
Read out the complete blog at http://blogs.technet.com/b/exchange/archive/2011/09/16/dag-beyond-the-a.aspx
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Designing a backup less Exchange 2010 Architecture
Step by step guide for upgrading Active Directory from Microsoft Windows 2003 to Microsoft Windows Server 2008
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Appear Offline in Microsoft Office Communicator Server 2007
Microsoft Exchange 2010 Test cases
Microsoft Exchange Server 2010 Disaster Recovery