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

[Ewan Milne]

Hi John-

Sorry, I was out for a couple of weeks and have been catching up...

Re: sg support, there were issues in the past with people attempting
to do SG_IO through dm-mp
assuming that DM would handle retry on other paths, which it didn't.
You also have to be aware
that non-idempotent commands don't work right if retried.  My
recommendation would be to avoid
implementing it, although there has been interest in a better way to
do multipathed "generic"
commands (e.g. virt pass-through) I think that is a more involved
project than you want to do here.

I see the discussion has progressed re: ALUA support in your later
patch postings, which is good.
As Hannes said, a Native SCSI MP would be useless without it.  You
don't have to support the
older non-ALUA mechanisms though, those arrays are way, way old.

SCSI does not have the equivalent of NVMe's AEN, so you need a way to
ensure that your
ALUA info is up-to-date.  DM-MP's path checker normally does this by
sending commands on
which the Unit Attention can be reported so that the code can fetch
up-to-date ALUA info.
Hannes made some optimizations years ago to avoid excessive RTPG
commands with large
numbers of LUNs which we would need also.

It will be necessary for the functionality to be enabled via a module
option, at least initially.
Introducing this in general use will be a big change for people who
have Enterprise SAN
configurations with their own custom path monitoring tools.  I believe
we put some functionality
into usespace multipath tools so e.g. Native NVMe devices can still be
monitored/observed
which made things a bit easier for people.

Unfortunately I will not be able to attend LSF/MM this year.  I am
sure it will be a good discussion.

-Ewan

On Wed, Feb 25, 2026 at 4:27 AM John Garry <john.g.garry at oracle.com> wrote:
>
> On 25/02/2026 08:11, Hannes Reinecke wrote:
> > And I _still_ want to have a blktests for persistent reservations ...
> nvme/054 supports resv testing.
>
> For scsi PR, we could use util-linux, which has blkpr.
>
>
>