[dm-devel] an interesting note on LUN coalescing

[Christophe Varoqui]

Hi,

you could also try to extend your observation to the timing of the udev
triggers.

You did not describe the scenario leading to this misbehaviour, but if it
involves paths remove-readd it's critical udev completed the /dev/ entries
janitoring ... unless multipathd might exec scsi_id on the wrong devices.

Latest distro versions implement a strict serialization, using the udevd
socket, thanks to Hannes work. You might try to reproduce the problem with
those to get a broader viewpoint, but it sure won't help with your
supporting your products on older distros.

Best regards,
Christophe Varoqui


On Thu, May 15, 2014 at 10:40 PM, Stewart, Sean <Sean.Stewart at netapp.com>wrote:

> On Thu, 2014-05-15 at 10:23 -0700, Brian Bunker wrote:
> > I will get both the page 0x83 and page 0x80 data of the path when the
> problem happens. In our case the page 0x83 and page 0x80 data are the same
> always. The page 0x83 data has the additional encoding and OUI information
> but the bytes after that 3624a9370 are exactly the same. I am wondering
> about how the path gets its wwid when it comes back.
>
> This is through the call to /lib/udev/scsi_id, which pulls it from page
> 0x83.  The function that does it is called get_uid().  It calls the
> external program, then strips trailing whitespace, replacing spaces with
> null characters.  You could try simply making the log message in that
> function a level 2 instead of a 3, so we'll know that get_uid completed
> for the path, and what it found for the uid.
>
> >
> > In our case we have a controller coming back online after doing
> something. All the LUNs are re-discovered by the initiator and an sd device
> gets created for each LUN path which came back. The serial of the sd device
> at that time I believe is known to the initiator. The multipathd gets that
> sd device and comes up with a wwid. I would expect that wwid would be just
> the serial number plus some bytes in the beginning but that is not what we
> see in the fail case. The path wwid and path serial are pointing to
> different volumes on the array. I expect that the sd device will show the
> same data for page 0x83 and page 0x80 pointing to the same volume on the
> array but I will verify that with more logging.
> >
> Similar to what I said above, you could change the log message in
> scsi_ioctl_pathinfo (after the get_serial call) from level 3 to level 2.
>
> Beyond that, I don't have many more ideas here..  Another wild guess, I
> suppose it's possible that the data from the 0x83 inquiry at the time of
> discovery could be incomplete?  Then, when you run the command later,
> manually, after it comes up, it appears just fine. I don't think I see
> the code zeroing the memory for the wwid in the newly allocated path
> struct before it tries to populate the buffer, but that seems kind of
> farfetched.
>
> > You can see from my previous posts on this issue. (They go back for some
> time). That the page 0x83 and page 0x80 from sg3_utils show that they point
> to same LUN but different from the dm device that they are coalesced under.
> >
> > Thanks,
> > Brian
>
> Hope this helps.
>
> Thanks,
> Sean
>
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