Kafka UnderMinIsrPartitionCount: confirming the write path is blocked

UnderMinIsrPartitionCount is non-zero and acks=all producers are failing. Unlike UnderReplicatedPartitions, which only opens a durability window, this metric confirms the broker is actively rejecting writes. It counts leader partitions where the in-sync replica set has shrunk below min.insync.replicas. In steady state it must be zero.

This article explains the metric, when to PAGE, and the exact checks to run when it fires.

What this means

UnderMinIsrPartitionCount is exposed per broker via JMX:

# Check current value on a broker
echo "get -b kafka.server:type=ReplicaManager,name=UnderMinIsrPartitionCount Value" | java -jar jmxterm.jar -l localhost:9999

This is a ReplicaManager metric: it counts only partitions where this broker is the leader. Aggregate across all brokers for a cluster-wide view. Any value above zero means at least one leader partition on this

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