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[dm-devel] Re: [PATCH RFC] move scsi parts of dm hw handlers to scsi layer

Rumor has it that on Fri, Jul 21, 2006 at 01:41:28PM +0200 Hannes Reinecke said:
> IIRC this is
> 'DGC' 'DISK'
> 'DGC' 'RAID 10'
> 'DGC' 'RAID 5'
> Hrmph. There is one bit which doesn't quite work out.
> While the hardware handler knows how to handle error codes and how to
> switch
> paths for a specific device, it doesn't know _when_ to switch it.
> I don't think it's a clever idea to switch paths whenever you encounter
> an
> passive path. Seems like you could do a nice ping-pong that way ...

Yes. I agree that having the handlers in SCSI is a better place for
them. However, the architectural decision to have multipath knowledge
at a higher level makes that harder. Some sort of interface into 
the scsi layer handlers from the dm/block layer would be more needed. 
That way dm or some piece that has the knowledge of the multipath 
situation can trigger the failover. 

There will still be a need to bubble up information from the decoded 
error codes so higher layers can make the correct decision. 
Such an interface would need to be in both directions.


> Cheers,
> Hannes
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Philip R. Auld, Ph.D.  	        	       Egenera, Inc.    
Software Architect                            165 Forest St.
(508) 858-2628                            Marlboro, MA 01752

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