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

[Martin Wilck]

Hello,

[sorry for cross-posting, I think this is relevant to multiple
communities.]

I'm referring to the recent discussion about SCSI device identification
for multipath-tools 
(https://listman.redhat.com/archives/dm-devel/2021-March/msg00332.html)

As you all know, there are different designators to identify SCSI LUNs,
and the specs don't mandate priorities for devices that support
multiple designator types. There are various implementations for device
identification, which use different priorities (summarized below).

It's highly desirable to clean up this confusion and settle on a single
instance and a unique priority order. I believe this instance should be
the kernel.

OTOH, changing device WWIDs is highly dangerous for productive systems.
The WWID is prominently used in multipath-tools, but also in lots of
other important places such as fstab, grub.cfg, dracut, etc. No doubt
that we'll be stuck with the different algorithms for years, especially
for LTS distributions. But perhaps we can figure out a long-term exit
strategy?

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.

In principle, I believe the kernel is "right" to prefer type 8. But
because the "wwid" attribute isn't actually used for device
identification today, changing the kernel logic would be less prone to
regressions than changing user space, even if it violates the principle
that the kernel's user space API must remain stable.

Would it be an option to modify the kernel logic?

If we can't, I think we should start with making the "wwid" attribute
part of the udev rule logic, and letting distros configure whether the
kernel logic or the traditional udev logic would be used.

Please tell me your thoughts on this matter.

Regards,
Martin

PS: Incomplete list of algorithms for SCSI designator priorities:

The kernel ("wwid" sysfs attribute) prefers "SCSI name string" (type 8)
designators over other types
(https://elixir.bootlin.com/linux/latest/A/ident/designator_prio).

The current set of udev rules in sg3_utils
(https://github.com/hreinecke/sg3_utils/blob/master/scripts/55-scsi-sg3_id.rules)
don't use the kernel's wwid attribute; they parse VPD 83 and 80
instead and prioritize types 36, 35, 32, and 2 over type 8.

udev's "scsi_id" tool, historically the first attempt to implement a
priority for this, doesn't look at the SCSI name attribute at all:
https://github.com/systemd/systemd/blob/main/src/udev/scsi_id/scsi_serial.c

There's a "fallback" logic in multipath-tools in case udev doesn't
provide a WWID:
https://github.com/opensvc/multipath-tools/blob/a41a61e8482def33e3ca8c9e3639ad2c37611551/libmultipath/discovery.c#L1040

-- 
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