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

[Benjamin Block]

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-muneendra.kumar@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.

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.

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@suse.de/

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


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