Re: [dm-devel] [PATCH 0/2] 'default' hardware handler for multipath

[Hannes Reinecke <hare@suse.de>]

On 04/03/2012 11:12 PM, Moger, Babu wrote:
> Thanks Hannes. We appreciate your work on this.
> 
>> -----Original Message-----
>> From: dm-devel-bounces@redhat.com [mailto:dm-devel-
>> bounces@redhat.com] On Behalf Of Hannes Reinecke
>> Sent: Monday, April 02, 2012 11:44 AM
>> To: linux-scsi@vger.kernel.org
>> Cc: dm-devel@redhat.com; Mike Snitzer
>> Subject: [dm-devel] [PATCH 0/2] 'default' hardware handler for multipath
>>
>> This patchset introduces a 'default' hardware handler for dm-multipath.
>> Modern storage arrays typically support two failover methods, the original
>> proprietary and the modern ALUA-based one.
>> The device_handler implementation will currently select the ALUA handler,
>> and falling back to the proprietary one if ALUA isn't supported.
>> However, in the built-in hardware table for multipath one can specify only
>> one hardware handler, causing the original hardware handler to be
>> overwritten.
>> By specifying a 'default' hardware handler multipath will not try to attach a
>> specific hardware handler, but rather using the currently attached on.
> 
> I think we still have some issues here. Right now we load the driver
> either by adding it in initrd or by using request_module call from device mapper.
> If the user passes, hardware_handler   "1 default"  from multipath then request_module will fail. 
> How are we going to load the driver if these handlers are not loaded. 
> 
The idea of the 'default' hardware handler is to avoid multipath
overriding the kernel matchine algorithm. To make this work we
obviously have to load the modules prior to start multipathing.

Given that we (ie RH and us) are already loading the device-handler
modules from the initrd independent from multipathing we should be fine.

But thinking a bit more about it, it might be better to handle this
via features.
Implementing a feature like 'default_hw_handler' would not override
the default hardware handler from the hardware table.
So the requested hardware handler would still be loaded, but the
actual initialisation would be controlled via the feature.

Yep, that sound better.
I'll be preparing a patch.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)
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