Re: [LSF/MM/BPF TOPIC] Multipath bio vs. request

[Mike Snitzer <snitzer@kernel.org>]

On Mon, Mar 24, 2025 at 04:20:12PM -0400, Mike Snitzer wrote:
> Hi John,
> 
> On Sat, Mar 22, 2025 at 02:38:29PM -0400, John Meneghini wrote:
> > I will be presenting on this topic at LSF/MM/BPF this year, in the IO track.
> > 
> > Here's an introduction for my talk.
> > 
> > DMMP currently supports two different kernel IO interfaces: the BIO interface[1] (struct bio) and the Request interface[2] (struct request).
> > By default DMMP uses the Request interface and over the years much work has been done test and improve the performance of the DMMP Request
> > interface. DMMP can also be manually configured to use the BIO interface. The DMMP BIO interface is supported but little work has been done
> > to test and improve its performance. DMMP is currently the only upstream component which continues to use the Request interface for submitting IO.
> 
> As I clarified at lunch today, your "DMMP is currently the only
> upstream component which continues to use the Request interface for
> submitting IO." makes no sense to me.  The request-based DM multipath
> target is a blk-mq driver.  It just acts like most blk-mq drivers.
> 
> What is different is DM core's request-based code will clone each
> request that gets submitted to the request-based DMMP device.  And
> then when the request is submitted to an underlying path it gets
> directly inserted in the unlering blk-mq request-queue for that path.

Sorry for typoe: s/unlering/underlying/
 
> So in those aspects request-based DM core and DM multipath are unique
> and they do require block interfaces that only benefit DMMP -- but
> that has _always_ been the case (nothing else ever needed to clone
> requests before submitting them).
> 
> > At the ALPSS 2024 conference last October we discussed the possibility of deprecating and eventually removing support the Request interface
> > as kernel API. Such a change could impact DMMP so I was asked if Red Hat would be willing to support the effort by measuring the performance
> > of DMMP's BIO interface[3] and comparing it to its Request based performance. Having such a comparative performance analysis would be very helpful
> > in determining what further changes might be needed to move DMMP away from using the Request interface. This would help with the overall effort
> > to improve BIO interface performance and eventually remove support for Request based IO as a kernel API.
> > 
> > In this presentation I will share the preliminary results of Red Hat's DMMP BIO vs Request performance tests[4] and discuss what the next possible
> > steps could be for moving forward.
> > 
> > The tests and performance graphs in this presentation were developed and run by Samuel Petrovic <spetrovi@redhat.com>.
> > Credit goes to Samuel for creating these performance tests and many thanks to Benjamin Marzinski <bmarzins@redhat.com>,
> > Mikulas Patocka <mpatocka@redhat.com> and others on the Red Hat DMMP and Performance teams who contributed to this work.
> > 
> > [1] https://lwn.net/Articles/736534/
> > [2] https://lwn.net/Articles/738449/
> > [3] https://lore.kernel.org/linux-scsi/643e61a8-b0cb-4c9d-831a-879aa86d888e@redhat.com
> > [4] https://people.redhat.com/jmeneghi/LSFMM_2025/DMMP_BIOvsRequest/
> 
> Other useful context is the 2007 paper that provides an overview of
> why dm-multipath was switched from bio-based to request-based:
> https://www.kernel.org/doc/ols/2007/ols2007v2-pages-235-244.pdf
>