[systemd-devel] Bug? /dev/disk/by-path symlinks disappear for iSCSI targets

[Kay Sievers]

On Tue, Oct 7, 2014 at 6:21 PM, Lee Duncan <lduncan at suse.com> wrote:
> I am debugging a problem where the symlinks in /dev/disk/by-path
> disappeared for iSCSI target devices.
>
> It looks like it's from systemd/udev commit
> e98bbfd2074e2b1079b7059341eac25741baf319
>
>  udev: path_id - suppress ID_PATH for devices with an unknown parent device type
>
> I believe the worry was that if you allowed pathnames based on a
> parent bus that did not supply unique IDs, then you could end up with
> duplicate paths, since this references a bug:
>
>     https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1321816
>
> But, looking at the code, this change seems to have assumed SCSI was
> not a "supported parent". I am not aware of any cases where SCSI has
> given duplicate names to devices

It is not about non-conflicting numbers, it is about predictability of
the numbers. The SCSI enumeration numbers are not predictable and
depend on enumeration or discovery order. The numbers used in by-path
links would need to be able to be calculated before the device is
connected.

The by-path links with non-predictable numbers are pointless and wrong
to create. It is expected that only a very limited subset of devices
support by-path, there is nothing wrong if iSCSI does not have them.

While we are at it: all specialized SCSI code should move to its own
package and out of the systemd/udev source tree. We will not extend
any SCSI handling in udev, we just cannot support it. Hannes already
started to work on moving things to sg3_utils. Hannes, any update on
this?

Thanks,
Kay