[dm-devel] [LSF/MM ATTEND] discuss blk-mq related to DM-multipath and status of XCOPY
Hannes Reinecke
hare at suse.de
Thu Jan 8 06:31:18 UTC 2015
On 01/08/2015 12:39 AM, Martin K. Petersen wrote:
>>>>>> "Hannes" == Hannes Reinecke <hare at 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
hare at suse.de +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
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