whether inter.broker.protocol.version could downgrade

[Xiangyuan LI <fl...@gmail.com>]

Hi,
    recently our team had try to upgrade kafka 0.10.0.1 to 2.2.0.we do so
by upgrade guide http://kafka.apache.org/documentation.html#upgrade, after
we set all brokers inter.broker.protocol.version to 2.2.0,we found that
producer produce rate become very slow and many messages take more than
500ms(even 1s,2s) to execute send method.
    we try to add max.in.flight.requests.per.connection,batch.size
params,but no use.
    because document said "Once the brokers begin using the latest protocol
version, it will no longer be possible to downgrade the cluster to an older
version.",so we have to start another kafka cluster and lose some messages.
    after that end,i try to upgrade a test cluster has 2 borkers and
version is 0.10.0.1, up to 2.2.0,and i change inter.broker.protocol.version
from 0.10.0.1 to 2.2.0,and then downgrade to 0.10.0.1 ,i found producer and
consumer hasn't any error during whole procedure.
     so now i have 2 questions:
     1.Is there some tips that why our producer send rate become slow?
     2.Is it really that inter.broker.protocol.version couldn't
downgrade?Or when we couldnt do so?

    thx..