[LSF/MM/BPF TOPIC] Native SCSI multipath support

[Hannes Reinecke]

On 2/14/26 10:42, John Garry wrote:
> On 13/02/2026 17:21, Hannes Reinecke wrote:
>>> At ALPSS 25 I presented a proposal for Native SCSI multipath support. 
>>> Let's discuss this topic at LSFMM.
>>>
>>> The idea for this is that SCSI could natively support multipath, like 
>>> how NVMe host driver does today. It is intended as an alternative to 
>>> dm- multipath support.
>>>
>>> I have been working on the implementation and I plan to post patches 
>>> in the next cycle. I am looking at a 3-stage approach:
>>> a. create a driver-agnostic multipath library, very heavily based on 
>>> NVMe host multipath support.
>>> The library would support features such as path management, path 
>>> selection/iopolicy, failover recovery, PR, delayed removal, gendisk 
>>> management etc.
>>> b. switch NVMe over to use this library
>>> c. add native SCSI multipath support based on this common library
>>>
>> Go for it, John!
>>
>> I'd be very interested in that.
> 
> cheers, in the meantime, I have some comments:
> 
> - I need to test PRs for both NVMe and SCSI, any advice on that would be 
> good. I don't think that blktests covers it. I did see Christoph mention 
> a testsuite at: https://lore.kernel.org/linux-nvme/1438672271-11309-1- 
> git-send-email-hch at lst.de/ - I can check that.
> 
Well, you might have seen the discussion on the device-mapper list, 
where stefanha is implementing generic PRs for dm-multipathing.
Or rather, trying to. We might need to revisit that and see what we
could be doing on the SCSI side.
Maybe we should be having a session about PRs at LSF?

> - I am still not sure on whether we require a multipath version of sg. 
> We can still have per-path sg. NVMe does have a multipath nvme-generic 
> dev, but that just handles IOCTLs/uring cmd, and nothing like sg read/ 
> write fops
> 
'sg' is primarily for testing 'raw' SCSI commands. (And dastardly 
complex to boot). I really would keep it in it's current form, and not
try to mimick something with SCSI multipathing.

> - I have not tried to detangle ALUA support from SCSI DH, so no ALUA 
> support yet
> 
Ouch. But that is the key point of the implementation; ALUA provides
_all_ the information required for multipathing, so how can you _not_
have support for it?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
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