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1. Set message.format.version=0.10.0 on one of the brokers (broker B).
2. Broker B will start to act like what described in phase 2.
   
   1. It will send FetchRequest V2 to other brokers for replication.
   2. When it sees FetchRequest V2 from other brokers and clietns, it will use zero copy
   3. When it sees It will only see ProduceRequest/ FetchRequest V1 from other brokers and clietns.clients, it will NOT use zero copy but do down-conversion
      
3. If something goes wrong, we can do the following to rollback:
   1. shutdown broker B
      
   2. nuke the data of the topics it was serving as leader before shutdown
      
   3. set message.format.version=0.9.0
   4. restart the broker to let the broker replicate from leaders. At this point the data on disk will be in MessageAndOffset V0.
      

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