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Note that I/O and file counts are really about #partitions/#brokers, so adding brokers will fix problems there; but zookeeper handles all partitions for the whole cluster so adding machines doesn't help.

Why do I see lots of Leader not local exceptions on the broker during controlled shutdown?

This happens when the producer clients are using num.acks=0. When the leadership for a partition is changed, the clients (producer and consumer) gets an error when they try to produce or consume from the old leader when they wait for a response. The client then refreshes the partition metadata from zookeeper and gets the new leader for the partition and retries. This does not work for the producer client when ack = 0. This is because the producer does not wait for a response and hence does not know about the leadership change. The client would end up loosing messages till the shutdown broker is brought back up. This issue is fixed in KAFKA-955