Basically, the procedure is straightforward and should be followed per DAG member:
- Appoint (next) DAG member;
- Move away all active copies on that DAG member;
- Prevent copies from activating on that DAG member;
- Perform maintenance, e.g. down or upgrade DAG member;
- Enable possible activation on that DAG member again;
- Optionally redistribute database copies.
Note that in a DAG configuration with 2 members, you need to be aware that during maintenance you have a temporary situation with no fail-over options. If that’s undesirable, consider implementing a 3rd DAG member.
To make the above procedure easier and automated regarding moves and activation (un)blocking, additional scripts are available since SP1 for Exchange 2010. These scripts are located in the Scripts folder, below the Exchange installation folder. By default the location of the scripts will be C:\Program Files\Microsoft\Exchange Server\v14\Scripts.
Utilizing them, the procedure is quite easy as you can see below. Note that the example uses a DAG named DAG1 with nodes ex2010a and ex2010b as members. They both host 2 databases, ex2010mdb1 and ex2010mdb2; both host 1 active copy and a passive copy of the other database.
- Appoint (next) DAG member, e.g. ex2010a;
- Run StartDagServerMaintenance.ps1 targeting that DAG member, e.g.:
.\StartDagServerMaintenance.ps1 –server ex2010a
- Perform maintenance;
- Run StopDagServerMaintenance.ps1 targeting that DAG member, e.g.:
.\StopDagServerMaintenance.ps1 –server ex2010a
- Repeat steps 2-3 for the other DAG member(s):
- Optionally run RedistributeActiveDatabases.ps1 for the DAG, e.g.:
.\RedistributeActiveDatabases.ps1 –DagName DAG1 –BalanceDBsByActivationPreference –Confirm:$false
Be advised that when upgrading on major levels (RTM to SP1 or SP1 to SP2), you can’t move a database to a lower level host. This means that when upgrading a node from SP1 to SP2 and moving a database to that SP2 node in the process, you can’t move that database to any SP1 nodes in the DAG. Keep this in mind when planning your upgrade, because it will impact the availability level by limiting your fallback options, albeit temporarily.