linux-kernel - Re: Regression in v4.6-rc due to SCSI multipath change

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Date:	Wed, 4 May 2016 10:59:20 +0200
From:	Hannes Reinecke <hare@...e.de>
To:	Paul Mackerras <paulus@...abs.org>
Cc:	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Regression in v4.6-rc due to SCSI multipath change

On 05/04/2016 08:51 AM, Paul Mackerras wrote:
> Current upstream kernels fail to boot on my POWER8 server with
> multipath SCSI disks and IPR host bus adapters.  What happens is that
> the system finds each disk twice (as normal) and then prints messages
> like this:
> 
> [    2.827761] sd 1:2:4:0: alua: supports implicit TPGS
> [    2.827875] sd 1:2:4:0: alua: No device descriptors found
> [    2.827923] sd 1:2:4:0: alua: Attach failed (-22)
> [    2.827979] device-mapper: table: 253:0: multipath: error attaching hardware handler
> [    2.828048] device-mapper: ioctl: error adding target to table
> 
> Eventually dracut times out (this is with Fedora 23) enters emergency
> mode.
> 
> I bisected the problem down to commit 0047220c6c36 ("scsi_dh_alua: use
> unique device id", 2016-02-19).  It seems that this commit adds the
> restriction that we can only do multipath with disks that have stuff
> in their VPD page 83 that scsi_vpd_lun_id() can parse.  The disks on
> my server apparently don't.
> 
> I instrumented scsi_vpd_lun_id() to find out what was going on.  The
> disks on this machine have a vendor-specific designator and a T10
> vendor ID based designator, but no designators of types 2, 3 or 8.
> An example from one disk is:
> 
> 02 01 00 20 49 42 4d 20 20 20 20 20 49 50 52 2d 30 20 20 20 35 45 43
> 34 41 42 30 30 30 30 30 30 30 30 32 30
> 
> 02 00 00 14 30 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20
> 
> I have a patch that extends scsi_vpd_lun_id() to be able to use the
> T10 vendor ID based designator, which fixes the problem on my system.
> I'll post the patch shortly.
> 
Please do. I'm about to draft a patch myself, but if you already
have one ...

> However, was it really intentional that multipath now can't be used
> with disks like these, when it worked just fine previously?
> 
Well. The thing is, ALUA can't really work if no VPD descriptors are
found, and so the check itself is correct.

Howver, we really need to parse all possible VPD descriptors, for
sure, so that is indeed a bug.
I'm preparing a patch for decoding all possible VPD descriptors, too.

Let's see who's first :-)

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		   Teamlead Storage & Networking
hare@...e.de			               +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
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