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

[Martin K. Petersen]

Martin,

> I suppose 99.9% of users never bother with customizing the udev rules.

Except for the other 99.9%, perhaps? :) We definitely have many users
that tweak udev storage rules for a variety of reasons. Including being
able to use RII for LUN naming purposes.

> 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.

That's fine with me.

> AFAICT, no major distribution uses "wwid" for this purpose (yet).

We definitely have users that currently rely on wwid, although probably
not through standard distro scripts.

> 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 like what NVMe did wrt. to exporting eui, nguid, uuid separately from
the best-effort wwid. That's why I suggested separate sysfs files for
the various page 0x83 descriptors. I like the idea of being able to
explicitly ask for an eui if that's what I need. But that appears to be
somewhat orthogonal to your request.

> 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).

That's fine. I am not a big fan of the idea that block/foo/wwid and
block/foo/device/wwid could end up being different. But I do think that
from a userland tooling perspective the consistency with NVMe is more
important.

-- 
Martin K. Petersen	Oracle Linux Engineering