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Note that followers will continue to write the records they receive from leaders without conversion. Since leaders will use v2 for new requests, replicating old data is the only case where v0 or v1 records may be written to disk after the upgrade to Apache Kafka 3.0 and the inter.broker.protocol.version is 3.0.

Produce and fetch requests with v0 and v1 message formats would be supported via up-conversion and down-conversion. Up-conversion and (especially) down-conversion have measurable performance impact due to increased CPU and memory usage, but the vast majority of Kafka clients have supported v2 for some time (even Spark, a notable late adopter, has supported v2 since Spark 2.4, which was released in October 2018).

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