A steady (unchanging) number of under-replicated partitions reported by many of the brokers in a cluster normally indicates that one of the brokers in the cluster is offline. The count of under-replicated partitions across the entire cluster will equal the number of partitions that are assigned to that broker, and the broker that is down will not report a metric. In this case, you will need to investigate what has happened to that broker and resolve that situation. This is often a hardware failure, but could also be an OS or Java issue that has caused the problem.
I wish I had read this a week ago or so... X-D
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