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:

  • The degree to which a system, subsystem, or equipment is in a specified operable and committable state at the start of a mission, when the mission is called for at an unknown, i.e., a random, time. Simply put, availability is the proportion of time a system is in a functioning condition. This is often described as a mission capable rate. Mathematically, this is expressed as 1 minus unavailability.
  • The ratio of (a) the total time a functional unit is capable of being used during a given interval to (b) the length of the interval.

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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