From mboxrd@z Thu Jan  1 00:00:00 1970
From: Mike Snitzer <snitzer@redhat.com>
Subject: Re: [PATCH 2/2] dm mpath: attach scsi_dh during table resume
Date: Thu, 25 Apr 2013 11:37:58 -0400
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To: "Bryn M. Reeves" <bmr@redhat.com>
Cc: Mikulas Patocka <mpatocka@redhat.com>, dm-devel@redhat.com, linux-scsi@vger.kernel.org, hare@suse.de

On Thu, Apr 25 2013 at 11:27am -0400,
Bryn M. Reeves <bmr@redhat.com> wrote:

> On 04/25/2013 03:50 PM, Mikulas Patocka wrote:
> >On Thu, 25 Apr 2013, Mike Snitzer wrote:
> >>The handler that is automatically attached _should_ be the correct
> >>handler.  We now have the .match() hook for scsi_dh and it has made for
> >>reliable scsi_dh attachment of the correct handler.
> >
> >The EMC devices work with both ALUA and EMC handlers - so there is no one
> >"correct" handler, the correct handler is the one that the user specified
> >in multipath configuration.
> 
> I think it's more absolute than that; if a Clariion array is in
> failover mode 4 (ALUA) then it's incorrect to use scsi_dh_emc and
> vice-versa.
> 
> The user can configure this in multipath.conf but it does not make
> it correct. The correct handler is the one that matches the
> configured failover mode of the array.
> 
> The ALUA handler scsi_device_tgps() in its match function but since
> the scsi_dh_emc match function only looks at the vendor/product it's
> impossible for it to make the correct decision.
> 
> The array can tell us what mode it's running in - teaching
> scsi_dh_emc to do this would seem to be an improvement.

clariion_match does more than check the vendor and product; if tpgs is
set (ALUA mode) it returns false.

So yes, while there is room for improvement in clariion_match the
current code should work just fine with reasoning between emc and alua.