From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id D1F29C54E71 for ; Wed, 21 May 2025 17:47:28 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1uHnWj-0002Hy-HP; Wed, 21 May 2025 13:46:50 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1uHnWh-0002H4-Sz for qemu-devel@nongnu.org; Wed, 21 May 2025 13:46:47 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1uHnWc-00072o-9f for qemu-devel@nongnu.org; Wed, 21 May 2025 13:46:47 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1747849599; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=GIVjUZ0G+0XzGnuQsGafc/1sAITX5npAyoV0HZIImwM=; b=ArHwVKpg2VwCvV5oiKye0sshN4gn6K7YWFDetmIQ7G4OIbsqwDyEIXM1lB4L/FZPYtKkUn 7vjfXekg6unXYdUCuj1efe+IyBzocGr20EGehfMv2Ap7rkTUxho3FDyrXOyt5p5gaDGtuW uQWXfL90Q2tpoZEVLzK5urXXbMam2Ds= Received: from mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-147-KFs8XNSZPkeQno7XB0K9Vw-1; Wed, 21 May 2025 13:46:37 -0400 X-MC-Unique: KFs8XNSZPkeQno7XB0K9Vw-1 X-Mimecast-MFC-AGG-ID: KFs8XNSZPkeQno7XB0K9Vw_1747849596 Received: from mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.40]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 14A03180045B; Wed, 21 May 2025 17:46:36 +0000 (UTC) Received: from redhat.com (unknown [10.45.226.112]) by mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id B39D019560B7; Wed, 21 May 2025 17:46:33 +0000 (UTC) Date: Wed, 21 May 2025 19:46:30 +0200 From: Kevin Wolf To: Stefan Hajnoczi Cc: qemu-block@nongnu.org, hreitz@redhat.com, pbonzini@redhat.com, bmarzins@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: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="1qwCXJTHJl0cxuYR" Content-Disposition: inline In-Reply-To: <20250520140339.GC82528@fedora> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.40 Received-SPF: pass client-ip=170.10.129.124; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, 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, T_SPF_TEMPERROR=0.01 autolearn=ham autolearn_force=no X-Spam_action: no action 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 --1qwCXJTHJl0cxuYR Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable 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 int= o the > > > > > > problem that the kernel dm-mpath doesn't know anything about SC= SI 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 accord= ing to > > > > > > the configured error policy. Obviously, this is not what users = want, > > > > > > they want working failover. > > > > > >=20 > > > > > > QEMU can parse the SG_IO result and determine whether this coul= d have > > > > > > been a path error, but just retrying the same request could jus= t send it > > > > > > to the same failing path again and result in the same error. > > > > > >=20 > > > > > > With a kernel that supports the DM_MPATH_PROBE_PATHS ioctl on d= m-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 pa= ths > > > > > > remained. If so, we can now retry the SG_IO ioctl and expect it= to be > > > > > > sent to a working path. > > > > > >=20 > > > > > > Signed-off-by: Kevin Wolf > > > > > > --- > > > > > > block/file-posix.c | 82 ++++++++++++++++++++++++++++++++++++++= +++++++- > > > > > > 1 file changed, 81 insertions(+), 1 deletion(-) > > > > >=20 > > > > > Maybe the probability of retry success would be higher with a del= ay so > > > > > that intermittent issues have time to resolve themselves. Either = way, > > > > > the patch looks good. > > > >=20 > > > > 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. > > >=20 > > > 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. > > >=20 > > > If the point is not to wait until some path comes back, then why loop= at > > > all? > >=20 > > 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. >=20 > 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? 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