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• The message.format.version controls the message format written to disk. The value equals to the magic byte of the message format. Introducing this configuration is to avoid doing version up/down conversion for the majority of users.
• If a consumer supports message.format.version, the broker will just use zero-copy transfer to send back the FetchResponse.
• If a consumer does not support message.format.version, the broker will have to do down conversion and send FetchResponse without using zero-copy.

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