[dm-devel] Re: [PATCH] RFC: have dm-mpath use already attached scsi_dh

[Hannes Reinecke]

Hi Mike,

Mike Christie wrote:
> Hannes Reinecke wrote:
>> Hi Mike,
>>
>> michaelc at cs.wisc.edu wrote:
>>> From: Mike Christie <michaelc at cs.wisc.edu>
>>>
>>> If you have a mixed environment of clarriions, where some
>>> support ALAU and some support PNR, what do you put in
>>> your multipath.conf? With this patch you do not have to worry about
>>> it. If those modules are loaded before dm-mpath, then they
>>> will have attached to the correct devices based on inquiry, alua
>>> commands
>>> and parsing of data buffers (for example in scsi_dh_emc's alua check).
>>> There is no need for the user to set that info in the multipath.conf.
>>> And in general since all scsi_dh_modules will attach to the devices
>>> they work for, we do not need to have users specific this.
>>>
>> No. The problem here is the hardware table from scsi_dh is compiled
>> in and cannot be changed from userland. The multipath.conf OTOH
>> is purely user-defined and, what's more, the user might have a valid
>> reason for modifying it.
>> (EG EMC Clariion can well be run in PNR mode even though ALUA is
>> active, or the user might want to try ALUA on any as-of-yet unknown
>> devices)
> 
> Ah. I misread the code and misunderstood the compat mode. I thought
> scsi_dh_emc was failing the attach when ALUA support was detected.
> 
>>
>> So _not_ allowing multipath to override the device handler setting
>> will just add to the confusion and makes error tracking even more
>> difficult.
>>
>> So I would prefer the attached patch, it even save to touch
>> device handler code at all.
>>
> 
> Thanks. I think this will work for me.
> 
> Are you going to push this for 2.6.30?
Well, yes, I could.

However, I'm still waiting for agk to push the request-based
multipath patches; I've got quite some updates here for multipathing
which are done with request-based multipath in place;
disentangling them will be quite time consuming (and pointless).

But this one, sure.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
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