This guide simply explains in a very easy way, all the technologies and procedures that you need to know to perform Exchange 2010 data center switch over, recovering DAG member or stretching DAG between sites.
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2. Datacenter Activation Coordination DAC
Active Manager handles DAC
DAC mode enables us to use three new commands: Start-DatabaseAvailabilityGroup, Stop-DatabaseAvailabilityGroup and Restore-DatabaseAvailabilityGroup
DAG uses DACP protocol to handle split brain scenarios when DAG is stretched to more than one subnet.
DAC when enabled, will be an extra application Quorum criteria.
DAC split DAG members to one of two sets:
- Stopped DAG Members – Command is Stop-DatabaseAvailabilityGroup
- Started DAG Members – Command is Start-DatabaseAvailabilityGroup
Only Started DAG Members will participate in DAC voting. Started servers are those candidate to bring their database copies online.
Stopped DAG member is the status of Active Manager that prevents the databases to be mounted on the server and will exclude it from DAC voting.
Note: So this might seems confusing to most of you. In simple words, when you enable DAC on your DAG, then it is not enough to have normal Cluster Quorum majority to bring databases online. Instead, we have to also test our servers for DAC.
2.2 How to get DAC OK status?
o If all started DAG members can communicate to each other
o If not, if a DAG Started member can communicate with a node with DAC bit 1
Note: In case of two DAG started members in the alternate datacenter exist, the boot time of the alternative witness share server can be used. If the witness boot time is before, DAC succeeded. Else, use Restore-DatabaseAvailabilityGroup . This only true for two member started DAG members.
In all cases, if all DAG members are DAC 0, use Start-DatabaseAvailabilityGroup to reset the DAC bit to 1 even if the nodes are already started.
Note: Again let me explain more. Suppose you have 5 DAG servers, SRV1 till SRV5. When you first turn all those servers together, then they will quickly have quorum majority and then will try to check if their DAC test is okay or not. The rule is simple, if all servers can communicate to each others, then each one will stamp itself with DAC value = 1 (Succeed).
Now suppose that SRV1 went down. When you bring it up, it will have DAC =0 and will try to do DAC test: Can SRV1 communicate with at least one server with DAC =1 ? Since SRV2 till SRV5 are all DAC=1, then SRV1 will assign it self with DAC=1 and will mount its databases.