[dm-devel] [PATCH 2/2] dm mpath: attach scsi_dh during table resume

[Mike Snitzer]

On Thu, Apr 25 2013 at 11:27am -0400,
Bryn M. Reeves <bmr at 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.