From patchwork Fri Sep 20 14:20:45 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Vladimir Sementsov-Ogievskiy X-Patchwork-Id: 1165222 Return-Path: X-Original-To: incoming@patchwork.ozlabs.org Delivered-To: patchwork-incoming@bilbo.ozlabs.org Authentication-Results: ozlabs.org; spf=pass (mailfrom) smtp.mailfrom=nongnu.org (client-ip=209.51.188.17; helo=lists.gnu.org; envelope-from=qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org; receiver=) Authentication-Results: ozlabs.org; dmarc=fail (p=none dis=none) header.from=virtuozzo.com 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 ozlabs.org (Postfix) with ESMTPS id 46ZbXN1Fysz9s00 for ; Sat, 21 Sep 2019 00:22:44 +1000 (AEST) Received: from localhost ([::1]:60142 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iBJo0-0004yT-Vi for incoming@patchwork.ozlabs.org; Fri, 20 Sep 2019 10:22:41 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:32808) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iBJmW-0004Z5-8a for qemu-devel@nongnu.org; Fri, 20 Sep 2019 10:21:09 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1iBJmU-0000tC-MZ for qemu-devel@nongnu.org; Fri, 20 Sep 2019 10:21:08 -0400 Received: from relay.sw.ru ([185.231.240.75]:43818) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1iBJmQ-0000gH-EY; Fri, 20 Sep 2019 10:21:02 -0400 Received: from [10.94.3.0] (helo=kvm.qa.sw.ru) by relay.sw.ru with esmtp (Exim 4.92.2) (envelope-from ) id 1iBJmM-0006b5-PW; Fri, 20 Sep 2019 17:20:58 +0300 From: Vladimir Sementsov-Ogievskiy To: qemu-block@nongnu.org Subject: [PATCH v13 04/15] block/backup: improve comment about image fleecing Date: Fri, 20 Sep 2019 17:20:45 +0300 Message-Id: <20190920142056.12778-5-vsementsov@virtuozzo.com> X-Mailer: git-send-email 2.21.0 In-Reply-To: <20190920142056.12778-1-vsementsov@virtuozzo.com> References: <20190920142056.12778-1-vsementsov@virtuozzo.com> MIME-Version: 1.0 X-detected-operating-system: by eggs.gnu.org: GNU/Linux 3.x X-Received-From: 185.231.240.75 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: fam@euphon.net, kwolf@redhat.com, vsementsov@virtuozzo.com, wencongyang2@huawei.com, xiechanglong.d@gmail.com, qemu-devel@nongnu.org, armbru@redhat.com, jsnow@redhat.com, stefanha@redhat.com, den@openvz.org, mreitz@redhat.com Errors-To: qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org Sender: "Qemu-devel" Signed-off-by: Vladimir Sementsov-Ogievskiy Reviewed-by: Max Reitz --- block/backup.c | 15 ++++++++++++--- 1 file changed, 12 insertions(+), 3 deletions(-) diff --git a/block/backup.c b/block/backup.c index 98d7f7a905..ae28849660 100644 --- a/block/backup.c +++ b/block/backup.c @@ -747,9 +747,18 @@ BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs, job->bitmap_mode = bitmap_mode; /* - * Set write flags: - * 1. Detect image-fleecing (and similar) schemes - * 2. Handle compression + * If source is in backing chain of target assume that target is going to be + * used for "image fleecing", i.e. it should represent a kind of snapshot of + * source at backup-start point in time. And target is going to be read by + * somebody (for example, used as NBD export) during backup job. + * + * In this case, we need to add BDRV_REQ_SERIALISING write flag to avoid + * intersection of backup writes and third party reads from target, + * otherwise reading from target we may occasionally read already updated by + * guest data. + * + * For more information see commit f8d59dfb40bb and test + * tests/qemu-iotests/222 */ job->write_flags = (bdrv_chain_contains(target, bs) ? BDRV_REQ_SERIALISING : 0) |