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

[Martin Wilck]

On Mo, 2021-07-05 at 15:11 +0200, Hannes Reinecke wrote:
> On 7/5/21 3:02 PM, Paolo Bonzini wrote:
> > On 02/07/21 16:21, Martin Wilck wrote:
> > > > SG_IO gives you raw access to the SCSI logic unit, and you get
> > > > to
> > > > keep the pieces if anything goes wrong.
> > > 
> > > That's a very fragile user space API, on the fringe of being
> > > useless
> > > IMO.
> > 
> > Indeed.  If SG_IO is for raw access to an ITL nexus, it shouldn't
> > be
> > supported at all by mpath devices.  If on the other hand SG_IO is
> > for
> > raw access to a LUN, there is no reason for it to not support
> > failover.
> > 
> Or we look at IO_URING_OP_URING_CMD, to send raw cdbs via io_uring.
> And delegate SG_IO to raw access to an ITL nexus.
> Doesn't help with existing issues, but should get a clean way
> forward.

I still have to understand how this would help with the retrying
semantics. Wouldn't we get the exact same problem if a path error
occurs?

Regards,
Martin