One of the exciting things about Exchange 2010 is the idea of the multi-role or "all-in-one" DAG server. This means having all three of the core Exchange 2010 roles installed on all of the servers in the DAG – Mailbox, Hub Transport and Client Access. How do we patch DAG?
- As always when moving databases around DAG, ensure that replication is healthy and all copies are up to date. For the purposes of ths example, we'll start upgrade process with Server 3.
- Activation block Server 3 so that a failure at this time won't activate copies of the mailbox databases on that server.
- Perform a switchover of all databases away from Server 3 to Server 1 and Server 3.
- Drain-stop all connections to CAS on Server 3, and then remove it from load-balanced array (note we don't remove server from CAS Array) and patch it.
- Add Server 3 back into load-balanced array, drain-stop Server 1 and Server 2 and remove them from load-balanced array.
- Notice for a short period, we've both upgraded and not-upgraded servers in the load-balanced array. This isn't an issue, because we still have all mailboxes on not-upgraded mailbox servers.
- Remove activation block on Server 3, activation block Server 2, perform switchover of all databases from Server 2 to Server 1 and Server 3, patch Server 2 and add it back into load-balanced array.
- Remove activation block on Server 2, activation block Server 1, perform switchover of all databases from Server 1 to Server 2 and Server 3, patch Server 1 and add it back into load-balanced array.
- Remove activation block on Server 1, and redistribute your databases evenly across all three servers (Note there'll be a pretty sweet script in Exchange 2010 SP1 to do this for you!).
More Info: Patching the Multi-Role Server DAG