[dm-devel] [PATCH v5 3/3] dm mpath: add CONFIG_DM_MULTIPATH_SG_IO - failover for SG_IO

[Martin Wilck]

On Do, 2021-07-01 at 13:34 +0200, Christoph Hellwig wrote:
> On Thu, Jul 01, 2021 at 12:35:53PM +0200, Martin Wilck wrote:
> > I respectfully disagree. Users of dm-multipath devices expect that
> > IO
> > succeeds as long as there's at least one healthy path. This is a
> > fundamental property of multipath devices. Whether IO is sent
> > "normally" or via SG_IO doesn't make a difference wrt this
> > expectation.
> 
> If you have those (pretty reasonable) expections don't use SG_IO.
> That is what the regular read/write path is for.  SG_IO gives you
> raw access to the SCSI logic unit, and you get to keep the pieces
> if anything goes wrong.

With this logic, if some paths are down, SG_IO commands on multipath
devices yield random results. On one path a command would fail, and on
another it would succeed. User space has no way to control or even see
what path is being used. That's a very fragile user space API, on the
fringe of being useless IMO.

If user space is interested in the error handling semantics you
describe, it can run SG_IO on individual SCSI devices any time. With
the change I am proposing, nothing is lost for user space. If user
space decides to do SG_IO on a multipath device, it has a different
expectation, which my patch set implements. IMO we should strive to
match the semantics for ioctls on natively multipathed NVMe namespaces.

Regards
Martin