Re: [dm-devel] [LSF/MM ATTEND] discuss blk-mq related to DM-multipath and status of XCOPY
On 01/04/2015 06:16 PM, Mike Snitzer wrote: > Hi, > > I'd like to attend LSF (and the first day of Vault). > > As a DM maintainer I'm open to discussing anything DM related. In > particular I'd like to at least have hallway track discussions with > key people interested in DM multipath support for blk-mq devices. > Keith Busch and I have gone ahead and implemented the bulk of this > request-based DM support for blk-mq devices, see: > https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/log/?h=dm-for-3.20-blk-mq > > But Bart says he is seeing inconsistent performance now that the code > appears relatively stable. I'll continue analyzing this aspect of > things and by the time LSF rolls around I hope the code to be sorted. > I'd be interested in having a discussion here, too. hch and me have discussed an alternative approach for multipathing (send the request directly and only clone request if the original submission failed) which I'd like to discuss with a broader audience. And I'd be interested in getting multipath to work with blk-mq, naturally. > Another topic we need to come to terms with is the state of XCOPY > (whether the initial approach needs further work, etc) -- Mikulas > Patocka reworked aspects of Martin's initial approach but it hasn't > progressed upstream: > https://www.redhat.com/archives/dm-devel/2014-October/msg00167.html > Yep. That definitely needs to be discussed. Especially we'd need to discuss how to handle exceptions, seeing that XCOPY might fail basically at any time. And there are some corner-cases (bandwidth starvation on the target) which needs to be discussed, too. Cheers, Hannes -- Dr. Hannes Reinecke zSeries & Storage h...@suse.de +49 911 74053 688 SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg) -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [dm-devel] [LSF/MM ATTEND] discuss blk-mq related to DM-multipath and status of XCOPY
> "Hannes" == Hannes Reinecke writes: Hannes> Yep. That definitely needs to be discussed. Especially we'd Hannes> need to discuss how to handle exceptions, seeing that XCOPY Hannes> might fail basically at any time. Like any SCSI command :) -- Martin K. Petersen Oracle Linux Engineering -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [dm-devel] [LSF/MM ATTEND] discuss blk-mq related to DM-multipath and status of XCOPY
On 01/07/2015 12:55 AM, Martin K. Petersen wrote: >> "Hannes" == Hannes Reinecke writes: > > Hannes> Yep. That definitely needs to be discussed. Especially we'd > Hannes> need to discuss how to handle exceptions, seeing that XCOPY > Hannes> might fail basically at any time. > > Like any SCSI command :) > Not quite. XCOPY is optional, and the system works well without it. So the exception handling would need to copy things by hand to avoid regressions. Plus XCOPY requires some elaborate setup, and even if those succeeded the array might still fail the command. _And_ there is no guarantee that that the XCOPY command is actually faster than the manual procedure. I guess we'll need some timing / initialisation routine to figure out what speed-up is to be had with XCOPY. Cheers, Hannes -- Dr. Hannes Reinecke zSeries & Storage h...@suse.de +49 911 74053 688 SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg) -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [dm-devel] [LSF/MM ATTEND] discuss blk-mq related to DM-multipath and status of XCOPY
> "Hannes" == Hannes Reinecke 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. -- Martin K. Petersen Oracle Linux Engineering -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [dm-devel] [LSF/MM ATTEND] discuss blk-mq related to DM-multipath and status of XCOPY
On 01/08/2015 12:39 AM, Martin K. Petersen wrote: >> "Hannes" == Hannes Reinecke 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 h...@suse.de +49 911 74053 688 SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg) -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [dm-devel] [LSF/MM ATTEND] discuss blk-mq related to DM-multipath and status of XCOPY
> "Hannes" == Hannes Reinecke writes: Hannes> But the array might prioritize 'normal' I/O requests, and treat Hannes> XCOPY with a lower priority. So given enough load XCOPY might Hannes> actually be slower than a normal copy ... Yes, but may have lower impact on concurrent I/O. -- Martin K. Petersen Oracle Linux Engineering -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
