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

[Benjamin Marzinski]

On Thu, Apr 22, 2021 at 09:07:15AM +0000, Martin Wilck wrote:
> On Wed, 2021-04-21 at 22:46 -0400, Martin K. Petersen wrote:
> > 
> > Martin,
> > 
> > > 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).
> > 
> > But user space can be adapted at runtime to pick one designator over
> > the
> > other (ha!).
> 
> And that's exactly the problem. Effectively, all user space relies on
> udev today, because that's where this "adaptation" is taking place. It
> happens
> 
>  1) either in systemd's scsi_id built-in 
>    (https://github.com/systemd/systemd/blob/7feb1dd6544d1bf373dbe13dd33cf563ed16f891/src/udev/scsi_id/scsi_serial.c#L37)
>  2) or in the udev rules coming with sg3_utils 
>    (https://github.com/hreinecke/sg3_utils/blob/master/scripts/55-scsi-sg3_id.rules)
> 
> 1) is just as opaque and un-"adaptable" as the kernel, and the logic is
> suboptimal. 2) is of course "adaptable", but that's a problem in
> practice, if udev fails to provide a WWID. multipath-tools go through
> various twists for this case to figure out "fallback" WWIDs, guessing
> whether that "fallback" matches what udev would have returned if it had
> worked.
> 
> That's the gist of it - the general frustration about udev among some
> of its heaviest users (talk to the LVM2 maintainers).
> 
> I suppose 99.9% of users never bother with customizing the udev rules.
> IOW, these users might as well just use a kernel-provided value. But
> the remaining 0.1% causes headaches for user-space applications, which
> can't make solid assumptions about the rules. Thus, in a way, the
> flexibility of the rules does more harm than it helps.
> 
> > We could do that in the kernel too, of course, but I'm afraid what
> > the
> > resulting BLIST changes would end up looking like over time.
> 
> That's something we want to avoid, sure.
> 
> But we can actually combine both approaches. If "wwid" yields a good
> value most of the time (which is true IMO), we could make user space
> rely on it by default, and make it possible to set an udev property
> (e.g. ENV{ID_LEGACY}="1") to tell udev rules to determine WWID
> differently. User-space apps like multipath could check the ID_LEGACY
> property to determine whether or not reading the "wwid" attribute would
> be consistent with udev. That would simplify matters a lot for us (Ben,
> do you agree?), without the need of adding endless BLIST entries.
> 

Yeah, as long as ID_LEGACY was changed in a careful manner, so WWIDs
didn't simply change without warning because of an upgrade, a path out
of this complexity is a definitely helpful.

-Ben

> 
> > I am also very concerned about changing what the kernel currently
> > exports in a given variable like "wwid". A seemingly innocuous change
> > to
> > the reported value could lead to a system no longer booting after
> > updating the kernel.
> 
> AFAICT, no major distribution uses "wwid" for this purpose (yet). I
> just recently realized that the kernel's ALUA code refers to it. (*)
> 
> In a recent discussion with Hannes, the idea came up that the priority
> of "SCSI name string" designators should actually depend on their
> subtype. "naa." name strings should map to the respective NAA
> descriptors, and "eui.", likewise (only "iqn." descriptors have no
> binary counterpart; we thought they should rather be put below NAA,
> prio-wise).
> 
> I wonder if you'd agree with a change made that way for "wwid". I
> suppose you don't. I'd then propose to add a new attribute following
> this logic. It could simply be an additional attribute with a different
> name. Or this new attribute could be a property of the block device
> rather than the SCSI device, like NVMe does it
> (/sys/block/nvme0n2/wwid).
> 
> I don't like the idea of having separate sysfs attributes for
> designators of different types, that's impractical for user space.
> 
> > But taking a step back: Other than "it's not what userland currently
> > does", what specifically is the problem with designator_prio()? We've
> > picked the priority list once and for all. If we promise never to
> > change
> > it, what is the issue?
> 
> If the prioritization in kernel and user space was the same, we could
> migrate away from udev more easily without risking boot failure.
> 
> Thanks,
> Martin
> 
> (*) which is an argument for using "wwid" in user space too - just to
> be consitent with the kernel's internal logic.
> 
> -- 
> 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
>