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This will be a silent breaking change since it flips the behavior around. 

Existing mirrormakers will be unaffected for any topics they are currently consuming since they already have a saved offset.

Mirrormakers that start consuming topics for which they don't have a saved offset Anyone who starts a mirrormaker on an existing topic will start replicating the partitions from the beginning, rather than from the partition's current highwater mark. If this behavior is unexpectedly applied to the mirrormakers starts consuming a very large partition/topic, it will replicate far more data than expected. Existing mirrormakers will be unaffected since they already have a saved offsetThis has relatively low probability since most of these topics are going to be newly-created topics anyway, so most of the time starting from the earliest simply prevents skipping the first few seconds/minutes of data written to the topic.

Since MirrorMaker 2.0 already behaves this way, this change will make future migrations from MM1 to MM2 easier for folks since the behavior will stop changing between them.

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