On 01/08/2015 12:39 AM, Martin K. Petersen wrote: >>>>>> "Hannes" == Hannes Reinecke <hare@xxxxxxx> 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@xxxxxxx +49 911 74053 688 SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg) -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
