[dm-devel] dm-multipath - IO queue dispatch based on FPIN Congestion/Latency notifications.

[Muneendra Kumar M]

Hi Benjamin,
My replies are below


On Tue, Mar 23, 2021 at 05:52:33PM +1000, Erwin van Londen wrote:
>> Hello All,
>>
> >Just wondering if there were any plans to incorporate FPIN
> >congestion/latency notifications in dm-multipath to disperse IO over
> >non-affected paths.
>

>For whats worth, general support in Kernel for a new path state in answer
to existing FPIN notifications was added earlier this year:
>https://lore.kernel.org/linux-scsi/1609969748-17684-1-git-send-email-mune
endra.kumar at broadcom.com/T/

>But this only adds a new port-state and support of it for one particular
driver (lpfc). Not aware of any other driver supporting this new state
yet, but I might have missed it. Also, the port-state is not set in
kernel, but has to be set by something external, unlike with RSCNs, where
we set the >state in the kernel.

We had a discussion with Marvel and they are adding the support in
their(qlaxx) driver.


>What it does, once a path is set into 'Marginal' state, is to not retry
commands on the same shaky path, once it already failed one time already.
Yes

>As far as dm-multipath is concerned, I asked that as well when this patch
series was developed:
>https://lore.kernel.org/linux-scsi/20201002162633.GA8365@t480-pf1aa2c2/
>Hannes answered that in the thread:
>https://lore.kernel.org/linux-scsi/ca995d96-608b-39b9-8ded-4a6dd7598660@s
use.de/

>Not sure what happened in between, didn't see anything on the mpath topic
yet.

As Hannes mentioned in his reply we have an external daemon called fctxpd
which acts on fpin-li events and sets the path to marginal path group as
well as set the port state to marginal.
This daemon is part of epel8.
Below is the path for the same where we have changes
https://github.com/brocade/bsn-fc-txptd

The above code is reviewed by the Benjamin Marzinski from redhat .

Note:The latest release will be available on the epel8 where we have the
support to set the port state to marginal in a week time

As we have all the support in the kernel for fpin registration,
notifications and also setting the port_state to marginal
We had a initial discussion with Hannes adding the fpin based native
support in dm multipathd for FPIN Congestion/Latency notifications .
I will take the initiative and start the discussion with Benjamin
Marzinski and get this work done with the help of Hannes.




Regards,
Muneendra.

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