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These cases all have one common root cause: the non-ISRs, which have not completed the log truncation for LMLE, can become the leader (due to unclean leader election), and it breaks the goal assumption at the beginning of this page:

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So, to resolve these cases, we introduce a new "topic-level" config mirror.support.unclean.leader.election . By default it is false. When it is true, during the log truncation for LMLE, we will wait for "all replicas" becomes ISR and completes the log truncation. Again, this is to fulfill the assumption above. So after the log truncation for LMLE, even if there is unclean leader election triggered any time, these log in 2 clusters can still converge successfully using the existing replication protocol.

If there are some replicas cannot catch up with the leader due to that are slow network, disk issue, ... etc while log truncation for LMLE, it will cause the mirroring pending or in FAILED state. In this case, users should manually resolve the issue, or re-assign the replicas into other healthy brokers, and start the mirroring again.