[systemd-devel] RFC: one more time: SCSI device identification

[Martin Wilck]

Hello Martin,

Sorry for the late response, still recovering from a week out of
office.

On Tue, 2021-04-06 at 00:47 -0400, Martin K. Petersen wrote:
> 
> Martin,
> 
> > The kernel's preference for type 8 designators (see below) is in
> > contrast with the established user space algorithms, which
> > determine
> > SCSI WWIDs on productive systems in practice. User space can try to
> > adapt to the kernel logic, but it will necessarily be a slow and
> > painful path if we want to avoid breaking user setups.
> 
> I was concerned when you changed the kernel prioritization a while
> back
> and I still don't think that we should tweak that code any further.

Ok.

> If the kernel picks one ID over another, that should be for the
> kernel's
> use. Letting the kernel decide which ID is best for userland is not a
> good approach.

Well, the kernel itself doesn't make any use of this property currently
(and user space doesn't much either, afaik).


> So I think my inclination would be to leave the current wwid as-is to
> avoid the risk of breaking things. And then export all ID descriptors
> reported in sysfs. Even though vpd83 is already exported in its
> entirety, I don't have any particular concerns about the individual
> values being exported separately. That makes many userland things so
> much easier. And I think the kernel is in a good position to
> disseminate
> information reported by the hardware.
> 
> This puts the prioritization entirely in the distro/udev/scripting
> domain. Taking the kernel out of the picture will make migration
> easier. And it allows a user to pick their descriptor of choice
> should a
> device report something completely unusable in type 8.

Hm, it sounds intriguing, but it has issues in its own right. For years
to come, user space will have to probe whether these attribute exist,
and fall back to the current ones ("wwid", "vpd_pg83") otherwise. So
user space can't be simplified any time soon. Speaking for an important
user space consumer of WWIDs (multipathd), I doubt that this would
improve matters for us. We'd be happy if the kernel could just pick the
"best" designator for us. But I understand that the kernel can't
guarantee a good choice (user space can't either).

What is your idea how these new sysfs attributes should be named? Just
enumerate, or name them by type somehow?

Thanks,
Martin

-- 
Dr. Martin Wilck <mwilck at suse.com>, Tel. +49 (0)911 74053 2107
SUSE Software Solutions Germany GmbH
HRB 36809, AG Nürnberg GF: Felix Imendörffer