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

[Hannes Reinecke]

On 4/27/21 10:10 AM, Martin Wilck wrote:
> On Tue, 2021-04-27 at 13:48 +1000, Erwin van Londen wrote:
>>>
>>> Wrt 1), we can only hope that it's the case. But 2) and 3) need work,
>>> afaics.
>>>
>> In my view the WWID should never change. 
> 
> In an ideal world, perhaps not. But in the dm-multipath realm, we know
> that WWID changes can happen with certain storage arrays. See 
> https://listman.redhat.com/archives/dm-devel/2021-February/msg00116.html 
> and follow-ups, for example.
> 
And it's actually something which might happen quite easily.
The storage array can unmap a LUN, delete it, create a new one, and map
that one into the same LUN number than the old one.
If we didn't do I/O during that interval upon the next I/O we will be
getting the dreaded 'Power-On/Reset' sense code.
_And nothing else_, due to the arcane rules for sense code generation in
SAM.
But we end up with a completely different device.

The only way out of it is to do a rescan for every POR sense code, and
disable the device eg via DID_NO_CONNECT whenever we find that the
identification has changed. We already have a copy of the original VPD
page 0x83 at hand, so that should be reasonably easy.

I had a rather lengthy discussion with Fred Knight @ NetApp about
Power-On/Reset handling, what with him complaining that we don't handle
is correctly. So this really is something we should be looking into,
even independently of multipathing.

But actually I like the idea from Martin Petersen to expose the parsed
VPD identifiers to sysfs; that would allow us to drop sg_inq completely
from the udev rules.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		        Kernel Storage Architect
hare at suse.de			               +49 911 74053 688
SUSE Software Solutions Germany GmbH, 90409 Nürnberg
GF: F. Imendörffer, HRB 36809 (AG Nürnberg)