Exchange 2010 introduced the database availability group (DAG), which enables you to design a mailbox resiliency configuration that’s essentially a redundant array of independent Mailbox servers. Multiple copies of each mailbox database are distributed across these servers to enable mailboxes to remain available during one or more server or database outages. As part of your design process, you need to design a balanced database copy layout, which may in turn, require you to revisit several design decisions to derive the optimal design. The following design principles should be used when planning the database copy layout:
Design Principle 1: Ensure that you minimize multiple database copy failures of a given mailbox database by isolating each copy from one another and placing them in different failure domains. A failure domain is a component or set of components that comprise a portion of the overall solution architecture (e.g., a server rack, a storage array, a router, etc.).