Re: [PATCH v1 0/2] Fix failover to non integrity NVMe path

[Max Gurtovoy <mgurtovoy@nvidia.com>]

On 24/04/2023 11:53, Sagi Grimberg wrote:
> 
>>> Yeah, I'm slightly unhappy with this whole setup.
>>> If we were just doing DIF I guess the setup could work, but then we 
>>> have to
>>> disable DIX (as we cannot support integrity data on the non-PI path).
>>> But we would need an additional patch to disable DIX functionality in 
>>> those
>>> cases.
>>
>> NVMeoF only supports inline integrity data, the remapping from out of
>> line integrity data is always done by the HCA for NVMe over RDMA,
>> and integrity data is not supported without that.
>>
>> Because of that I can't see how we could sensibly support one path with
>> integrity offload and one without.  And yes, it might make sense to
>> offer a way to explicitly disable integrity support to allow forming such
>> a multipath setup.
> 
> I agree. I didn't read through the change log well enough, I thought
> that one path is DIF and the other is DIX.
> 
> I agree that we should not permit such a configuration.

I'm not yet convinced why not to permit it.
The spec allows this to happen and I can think about scenarios that 
users will want this kind of configuration.

We also support it today (with the exception on this bug).
There is a special PRACT bit in the spec that asks the controller to 
take action upon each R/W IO request.

The Multipath is not related to md IMO. One path can generate/verify md 
and other can raise PRACT bit.

You can also create 2 paths from different hosts to same target and one 
will have ConnectX-5 and other ConnectX-3. The fact that these path are 
from the same host is not so important IMO.