First, let's define what split-brain is. Each replica can be either connected or disconnected towards to the other. If the replica spontaneously goes to StandAlone. It means that it refuses to accept the state and don't want to synchronize with the other. This is a classic split-brain situation.
Solving the split-brain for two replicas is done in the same way as for multiple replicas.
First, let's decide which replica we want to synchronize with. To do this, look into
drbdadm status <res>
If the replica we need is in the StandAlone or Outdated/Inconsistent state, it must first be switched to UpToDate. To acheve this go to the node with this replica and execute:
drbdadm primary --force <res>
drbdadm secondary <res>
And in order to make the rest of the replicas forget about their state and synchronize the data from the UpToDate replicas. Go to the nodes with them and execute:
drbdadm disconnect <res>
drbdadm connect <res> --discard-my-data
It is worth mentioning that in the latest versions of LINSTOR, the auto-tiebreaker function is enabled by default. This means, when you creating a resource in two replicas, automatically adds a third diskless replica, which is a kind of arbiter for ensuring the quorum. Thus, it is less and less usual to solve split-brain these days.
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