Digital Press 4 Exchange Server

Since Exchange 2010 was released during the winter of 2009, its high availability feature ie; DAG is the most talked feature.

The DAG differs from Exchange Server 2007 SP1 in the following ways:

►With CCR, there can be only two highly available copies of the database within
the cluster; within the DAG there can be up to 16 copies of each database.

►With SCR, the activation process required administrative intervention; within a DAG,
failover between individual database copies can happen automatically.

►With SCC, a single shared copy of the database consumes less storage but provides
no redundancy. Exchange Server 2010 has no configuration that replaces this
functionality, although some third-party solutions may be able to provide similar
functionality by using the Third Party Replication API.

►With LCR, a single-server configuration allows two copies of a database to reside
on different storage connected to the same server. No configuration in Exchange
Server 2010 replaces this functionality.

A DAG also has the following characteristics:

►Requires the Windows failover clustering feature and uses an Enterprise version of
Windows server (Windows Server 2008 or Windows Server 2008 R2), although the
installation and configuration tasks occur with the Exchange Server management tools.
Exchange Server does not use Windows failover clustering to handle database failover.
Instead, it uses Active Manager to manage the failover process.
Members must have the same operating system.

►You can add up to 16 servers to a single DAG and create up to 16 copies of a database.
Up to 100 databases can be mounted as either a passive or active copy of the database
on each server in the DAG.

►A DAG can be created after you install the Mailbox server. If a Mailbox server is hosting
active mailbox databases, it can be added to a DAG later, it if meets the requirements.
n Allows you to move a single database between servers in the DAG without affecting
other databases. Failover occurs per mailbox database, not for an entire server.
n Allows up to 16 copies of a single database on separate servers. A server can only host
one copy of each database.

►Requires the database and transaction log copies for each database to be stored
in the same path on all servers. For example, if you store Mailbox Database 1 in D:\DB\
Mailbox Database 1\ on Dallas-MB01A, you must also store it in D:\DB\Mailbox
Database 1\ on all other servers that host copies of Mailbox Database 1.

►Defines the boundary for replication, failovers, and switchovers—only servers in the
DAG can host database copies. You cannot replicate database copies to Mailbox
servers that are not in the same DAG.

►Does not require that all databases have the same number of copies. In a 16-node
DAG, one database can have 16 copies, whereas other databases are neither redundant
nor have varying number of copies.

The above points make DAG one of the mine personal favourite features on Exchange 2010..

Hope this info helps….