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Wed, 21 May 2025 18:23:31 +0000 (UTC) Received: from bmarzins-01.fast.eng.rdu2.dc.redhat.com (unknown [10.6.23.247]) by mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id EACE719560B7; Wed, 21 May 2025 18:23:30 +0000 (UTC) Received: from bmarzins-01.fast.eng.rdu2.dc.redhat.com (localhost [127.0.0.1]) by bmarzins-01.fast.eng.rdu2.dc.redhat.com (8.18.1/8.17.1) with ESMTPS id 54LINTBo597458 (version=TLSv1.3 cipher=TLS_AES_256_GCM_SHA384 bits=256 verify=NOT); Wed, 21 May 2025 14:23:29 -0400 Received: (from bmarzins@localhost) by bmarzins-01.fast.eng.rdu2.dc.redhat.com (8.18.1/8.18.1/Submit) id 54LINTQI597457; Wed, 21 May 2025 14:23:29 -0400 Date: Wed, 21 May 2025 14:23:29 -0400 From: Benjamin Marzinski To: Kevin Wolf Cc: Stefan Hajnoczi , qemu-block@nongnu.org, hreitz@redhat.com, pbonzini@redhat.com, qemu-devel@nongnu.org Subject: Re: [PATCH] file-posix: Probe paths and retry SG_IO on potential path errors Message-ID: References: <20250513113730.37404-1-kwolf@redhat.com> <20250513135148.GB227327@fedora> <20250515140142.GA333399@fedora> <20250520140339.GC82528@fedora> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.0 on 10.30.177.40 Received-SPF: pass client-ip=170.10.133.124; envelope-from=bmarzins@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -22 X-Spam_score: -2.3 X-Spam_bar: -- X-Spam_report: (-2.3 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.184, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-Mailman-Approved-At: Wed, 21 May 2025 15:58:39 -0400 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Wed, May 21, 2025 at 07:46:30PM +0200, Kevin Wolf wrote: > Am 20.05.2025 um 16:03 hat Stefan Hajnoczi geschrieben: > > On Thu, May 15, 2025 at 05:02:46PM +0200, Kevin Wolf wrote: > > > Am 15.05.2025 um 16:01 hat Stefan Hajnoczi geschrieben: > > > > On Thu, May 15, 2025 at 10:15:53AM +0200, Kevin Wolf wrote: > > > > > Am 13.05.2025 um 15:51 hat Stefan Hajnoczi geschrieben: > > > > > > On Tue, May 13, 2025 at 01:37:30PM +0200, Kevin Wolf wrote: > > > > > > > When scsi-block is used on a host multipath device, it runs into the > > > > > > > problem that the kernel dm-mpath doesn't know anything about SCSI or > > > > > > > SG_IO and therefore can't decide if a SG_IO request returned an error > > > > > > > and needs to be retried on a different path. Instead of getting working > > > > > > > failover, an error is returned to scsi-block and handled according to > > > > > > > the configured error policy. Obviously, this is not what users want, > > > > > > > they want working failover. > > > > > > > > > > > > > > QEMU can parse the SG_IO result and determine whether this could have > > > > > > > been a path error, but just retrying the same request could just send it > > > > > > > to the same failing path again and result in the same error. > > > > > > > > > > > > > > With a kernel that supports the DM_MPATH_PROBE_PATHS ioctl on dm-mpath > > > > > > > block devices (queued in the device mapper tree for Linux 6.16), we can > > > > > > > tell the kernel to probe all paths and tell us if any usable paths > > > > > > > remained. If so, we can now retry the SG_IO ioctl and expect it to be > > > > > > > sent to a working path. > > > > > > > > > > > > > > Signed-off-by: Kevin Wolf > > > > > > > --- > > > > > > > block/file-posix.c | 82 +++++++++++++++++++++++++++++++++++++++++++++- > > > > > > > 1 file changed, 81 insertions(+), 1 deletion(-) > > > > > > > > > > > > Maybe the probability of retry success would be higher with a delay so > > > > > > that intermittent issues have time to resolve themselves. Either way, > > > > > > the patch looks good. > > > > > > > > > > I don't think adding a delay here would be helpful. The point of > > > > > multipath isn't that you wait until a bad path comes back, but that you > > > > > just switch to a different path until it is restored. > > > > > > > > That's not what this loop does. DM_MPATH_PROBE_PATHS probes all paths > > > > and fails when no paths are available. The delay would only apply in the > > > > case when there are no paths available. > > > > > > > > If the point is not to wait until some path comes back, then why loop at > > > > all? > > > > > > DM_MPATH_PROBE_PATHS can only send I/O to paths in the active path > > > group, so it doesn't fail over to different path groups. If there are no > > > usable paths left in the current path group, but there are some in > > > another one, then the ioctl returns 0 and the next SG_IO would switch to > > > a different path group, which may or may not succeed. If it fails, we > > > have to probe the paths in that group, too. > > > > This wasn't obvious to me, can that be emphasized in the code via naming > > or comments? About retrying up to 5 times: is the assumption that there > > will be 5 or fewer path groups? > > Originally, the thought behind the 5 was more about the case where > DM_MPATH_PROBE_PATHS offlines bad paths, but then another one goes down > before we retry SG_IO, so that it fails again. > > But you're right that it would now apply to retrying in a different path > group. The assumption we make would then be that there will be 5 or > fewer path groups with no working path in them (rather than just 5 of > them existing). That doesn't seem like a completely unreasonable > assumption, but maybe we should increase the number now just to be on > the safe side? > > Ben, do you have an opinion on this? 5 seems like a reasonable number. Unless people have the path_grouping_policy set to failover, 5 path groups seems like more than enough. You could make the argument that if users were configured with failover (one path per path group) or had a number paths marked marginal (and placed into marginal path groups), you could exceed 5 path groups. 8 would seems like a reasonable maximum then. It is possible to have multipath devices with over 8 paths, but that's pretty rare. -Ben > > Kevin