[PATCH] nvme-rdma: set ack timeout of RoCE to 262ms

[Sagi Grimberg]

>>>> If so, which devices did you use ?
>>> The host HBA is Mellanox Technologies MT27800 Family [ConnectX-5];
>>> The switch and storage are huawei equipments.
>>> In principle, switches and storage devices from other vendors
>>> have the same problem.
>>> If you think it is necessary, we can test the other vendor switchs
>>> and linux target.
>>
>> Why is the 2s default chosen, what is the downside for a 250ms seconds 
>> ack timeout? and why is nvme-rdma different than all other kernel rdma
> The downside is redundant retransmit if the packets delay more than
> 250ms in the networks and finally reaches the receiver.
> Only in extreme scenarios, the packet delay may exceed 250 ms.

Sounds like the default needs to be changed if it only addresses the
extreme scenarios...

>> consumers that it needs to set this explicitly?
> The real-time transaction services are sensitive to the delay.
> nvme-rdma will be used in real-time transactions.
> The real-time transaction services do not allow that the packets
> delay more than 250ms in the networks.
> So we need to set the ack timeout to 262ms.

While I don't disagree with the change itself, I do disagree why this
needs to be driven by nvme-rdma locally. If all kernel rdma consumers
need this (and if not, I'd like to understand why), this needs to be set 
in the rdma core.