Re: [dm-devel] [LSF/MM ATTEND] discuss blk-mq related to DM-multipath and status of XCOPY

[Hannes Reinecke <hare@suse.de>]

On 01/08/2015 12:39 AM, Martin K. Petersen wrote:
>>>>>> "Hannes" == Hannes Reinecke <hare@suse.de> writes:
> 
> Hannes> Not quite. XCOPY is optional, and the system works well without
> Hannes> it.  So the exception handling would need to copy things by hand
> Hannes> to avoid regressions.
> 
> Or defer to user space.
> 
> But it's really no different from how we do WRITE SAME which may or may
> not work. If it fails we fall back to writing zeroes.
> 
> Hannes> Plus XCOPY requires some elaborate setup, and even if those
> Hannes> succeeded the array might still fail the command.  _And_ there
> Hannes> is no guarantee that that the XCOPY command is actually faster
> Hannes> than the manual procedure.
> 
> It saves the data a roundtrip on the fabric. That itself may be more
> valuable than a direct bandwidth win if there is concurrent I/O on the
> wire.
> 
But the array might prioritize 'normal' I/O requests, and treat XCOPY
with a lower priority. So given enough load XCOPY might actually be
slower than a normal copy ...

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
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