From mboxrd@z Thu Jan 1 00:00:00 1970 From: Hannes Reinecke Subject: Re: [dm-devel] [PATCH v2] scsi: Fix dm-multipath starvation when scsi host is busy Date: Tue, 22 May 2012 15:01:43 +0200 Message-ID: <4FBB8E37.60300@suse.de> References: <4FBB555B.5060608@ce.jp.nec.com> <1337677994.28492.12.camel@dabdike.int.hansenpartnership.com> <4FBB62FD.5040008@ce.jp.nec.com> <20120522125445.GA26233@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <20120522125445.GA26233@redhat.com> Sender: linux-scsi-owner@vger.kernel.org To: device-mapper development Cc: Mike Snitzer , Jun'ichi Nomura , Bernd Schubert , Alasdair G Kergon , linux-scsi List-Id: dm-devel.ids On 05/22/2012 02:54 PM, Mike Snitzer wrote: > On Tue, May 22 2012 at 5:57am -0400, > Jun'ichi Nomura wrote: >=20 >> On 05/22/12 18:13, James Bottomley wrote: >>> Isn't a more understandable explanation: >> >> Thank you. That's quite elegant. >> I replaced the description with it. >> >>> And please put a comment in the code as well otherwise someone will >>> eventually send a "fix" for this because we're not paying attention= to >>> host busy (and I'll have forgotten about the issue by then and migh= t >>> apply it). >> >> Added the comment in code. >> >>> A final note is that this is more a band aid than a fix because thi= s is >>> still a congestion situation dm-mp should be aware of. >> >> Yes. To do that, we have to generalize the concept of "host" >> and share it with block layer. >> >> Attached below is the revised patch. >> >> ---------------------------------------------------------- >> [PATCH v2] scsi: Fix dm-multipath starvation when scsi host is busy >> >> block congestion control doesn't have any concept of fairness across >> multiple queues. This means that if SCSI reports the host as busy i= n >> the queue congestion control it can result in an unfair starvation >> situation in dm-mp if there are multiple multipath devices on the sa= me >> host. For example: >> http://www.redhat.com/archives/dm-devel/2012-May/msg00123.html >> >> The fix for this is to report only the sdev busy state (and ignore t= he >> host busy state) in the block congestion control call back. >> The host is still congested, but the SCSI subsystem will sort out th= e >> congestion in a fair way because it knows the relation between the >> queues and the host. >> >> Reported-by: Bernd Schubert >> Tested-by: Bernd Schubert >> Signed-off-by: Jun'ichi Nomura >> Cc: James Bottomley >> Cc: Mike Snitzer >=20 > Acked-by: Mike Snitzer >=20 Acked-by: Hannes Reinecke 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