From mboxrd@z Thu Jan 1 00:00:00 1970 From: Hannes Reinecke Subject: Re: [dm-devel] [LSF/MM ATTEND] discuss blk-mq related to DM-multipath and status of XCOPY Date: Thu, 08 Jan 2015 07:31:18 +0100 Message-ID: <54AE2436.2000004@suse.de> References: <54AA90B1.1030201@suse.de> <54ACEF13.7000508@suse.de> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: Sender: linux-scsi-owner@vger.kernel.org To: "Martin K. Petersen" Cc: device-mapper development , Christoph Hellwig , "linux-scsi@vger.kernel.org" List-Id: dm-devel.ids On 01/08/2015 12:39 AM, Martin K. Petersen wrote: >>>>>> "Hannes" =3D=3D Hannes Reinecke writes: >=20 > Hannes> Not quite. XCOPY is optional, and the system works well witho= ut > Hannes> it. So the exception handling would need to copy things by h= and > Hannes> to avoid regressions. >=20 > Or defer to user space. >=20 > But it's really no different from how we do WRITE SAME which may or m= ay > not work. If it fails we fall back to writing zeroes. >=20 > Hannes> Plus XCOPY requires some elaborate setup, and even if those > Hannes> succeeded the array might still fail the command. _And_ ther= e > Hannes> is no guarantee that that the XCOPY command is actually faste= r > Hannes> than the manual procedure. >=20 > 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 th= e > wire. >=20 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 --=20 Dr. Hannes Reinecke zSeries & Storage hare@suse.de +49 911 74053 688 SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 N=FCrnberg GF: J. Hawn, J. Guild, F. Imend=F6rffer, HRB 16746 (AG N=FCrnberg) -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html