Re: mechanism for multipath to pass information to hardware handler

From: Mike Christie <michaelc@cs.wisc.edu>
To: sekharan@linux.vnet.ibm.com
Cc: SCSI development list <linux-scsi@vger.kernel.org>,
	asson_ronald <asson_ronald@emc.com>,
	"Rafael J. Wysocki" <rjw@sisk.pl>,
	device-mapper development <dm-devel@redhat.com>,
	Mike Christie <mchristi@redhat.com>,
	"Benoit, Arthur" <Benoit_Arthur@emc.com>
Subject: Re: mechanism for multipath to pass information to	hardware handler
Date: Sat, 27 Jun 2009 12:48:26 -0500	[thread overview]
Message-ID: <4A465B6A.5070507@cs.wisc.edu> (raw)
In-Reply-To: <1246061016.14164.45.camel@chandra-ubuntu>

On 06/26/2009 07:03 PM, Chandra Seetharaman wrote:
> On Fri, 2009-06-26 at 14:55 -0500, James Bottomley wrote:
>> On Fri, 2009-06-26 at 12:45 -0700, Chandra Seetharaman wrote:
>>> Yes, Mike Christie and I were aware of this and it was one of the issue
>>> we were trying to resolve before we pushed scsi_dh interface upstream.
>>> (It is little complicated as we need the parameters to be set per
>>> vendor-product tuple).
>>>
>>> The original code I ported to scsi_dh interface was from Ed Goggin(who
>>> was working for EMC then). IIRC, he was also aware of this issue.
>>>
>>> When we pushed scsi_dh interface, we did get few of the EMC folks (on
>>> Cc) to review/test the code and they did, and this issue was not seen as
>>> a problem.
>>>
>>> We wanted to get back to that issue sometime later, got busy with other
>>> things, and it disappeared from my list of things-to-do as the
>>> regression was not seen as an issue (till now :)... I will get back to
>>> it.
>> One way around this might simply be to make the device_handlers create a
>> sysfs interface for additional parameters.  Then the multipath command
>
> Yes, having a per device interface is one of the options we considered.
> Once we have that defined we can either use sysfs or a scsi_dh_8() API
> to achieve the same.
>
> But we wanted to have it per vendor-product instead of per device, I
> don't recall the problems associated with it. Mike(C), do you recall ?
>

I do not think you want the interface to be per vendor-product in the 
kernel. You might want some devices with some feature on and some with 
it off. In userspace we can decide if it is should be per vendor-product 
or per device, and then have multipath set things up. The kernel 
interface should probably be more generic.

>> can feed them (or in a pinch, users relying on the features can pass
>> them in manually).  Right at the moment having no possible work around
>> does appear to be an issue.
>
> I will try to roll up a patch soon.
>> James
>>
>>
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>
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