[PATCH 05/17] nvme: wire-up support for async-passthru on char-device.

[Sagi Grimberg]

>> [...]
>>
>>> Got it, thanks. Passthrough (sync or async) just returns the failure
>>> to user-space if that fails.
>>> No attempt to retry/requeue as the block path does.
>>
>> I know, and that was my original question, no one cares that this
>> interface completely lacks this capability? Maybe it is fine, but
>> it is not a trivial assumption given that this is designed to be more
>> than an interface to send admin/vs commands to the controller...
> 
> Most people don't really care about or use multipath, so it's not a
> primary goal.

This statement is generally correct. However what application would be 
interested in speaking raw nvme to a device and gaining performance that
is even higher than the block layer (which is great to begin with)?

First thing that comes to mind is a high-end storage array, where
dual-ported drives are considered to be the standard. I could argue the
same for a high-end oracle appliance or something like that... Although
in a lot of cases, each nvme port will connect to a different host...

What are the use-cases that need this interface that are the target
here? Don't remember seeing this come up in the cover-letter or previous
iterations...

> For passthrough, most of request types should hit the
> exact target, I would suggest that if someone cares about multipath for
> specific commands, that they be flagged as such.
What do you mean by "they be flagged as such"?