The next major version of Exchange, Exchange 2010, introduces a valuable new concept called a Database Availability Group (DAG). A DAG is group of up to 16 mailbox servers that use continuous replication to update database copies, communicate to manage failures that affect individual databases, and can provide automatic recovery from a variety of failures. A database copy is a single mailbox database with its own transaction logs. Storage Groups no longer exist in Exchange 2010. Exchange 2010 supports a maximum of 100 Mailbox Databases per server. Just like in the past, the size of each database is unlimited, but best practice is to limit size so recovery time is optimal.
DAGs are part of the Exchange 2010 high availability solution which now provides database-level failover. Each Mailbox Database can have up to 16 copies. Exchange will manage the replication of data automatically between the database copies to keep data consistent. Replication is based on the transaction log files, similar to how Exchange 2007 used logs files with LCR, SCR, and CCR, which by the way are discontinued in Exchange 2010. By having multiple copies of each database, traditional Exchange backup is no longer a necessity. Should a hardware failure occur, Exchange can simply mount one of the database copies. According to Microsoft, failover time is less than 30 seconds.
... Bob Spurzem
One Comment
Oct 29, 2009. We updated this bulletin. The previous version had some confusion between Database Copies and Database Availability Groups
–david
When you said: “By having multiple copies of each database, traditional Exchange backup is no longer a necessity.”
And about transaction logs? Circular loggin is the solution? Can I use circular loggin with DAG?