[gazarsgo]

There's no downtime in mirroring a cluster with MirrorMaker, and it's become common to isolate producers from consumers to separate clusters, see https://medium.com/netflix-techblog/kafka-inside-keystone-pi...

Ordering of messages is completely unaffected, it's the routing of future events that's affected when you increase partition count. This is critical for some use cases (windowing of data for analytics purposes, for example) but irrelevant to others.

Data duplication issues? This sounds like FUD also, but common guidance is to design your events to be idempotent, or utilize Kafka's new exactly-once delivery.

You can expand partition counts for a topic dynamically.

You can't currently decrease partition counts, because given the current design that could orphan both data and consumers.