@Override
  protected void prepareBuffer() {
    super.prepareBuffer();
    bufferToSend.writeShort(VERSION_V2);
    bufferToSend.writeLong(acceptableLagInBytes);
    bufferToSend.writeShort(numReplicasCaughtUpPerPartition);
  }
@return the least number of replicas that have to be within {@link #getAcceptableLagInBytes()} for each partition.
ambry-protocol/src/main/java/com/github/ambry/protocol/CatchupStatusAdminRequest.java