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 X-Spam-Level: X-Spam-Status: No, score=-9.7 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id CC0D3C47E48 for ; Thu, 26 Sep 2019 10:32:58 +0000 (UTC) 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 mail.kernel.org (Postfix) with ESMTPS id A6617222CE for ; Thu, 26 Sep 2019 10:32:58 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org A6617222CE Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=virtuozzo.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:33566 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iDR4z-0003Mf-0d for qemu-devel@archiver.kernel.org; Thu, 26 Sep 2019 06:32:57 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:38432) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iDQyh-0006e5-5P for qemu-devel@nongnu.org; Thu, 26 Sep 2019 06:26:28 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1iDQyf-00073m-M9 for qemu-devel@nongnu.org; Thu, 26 Sep 2019 06:26:26 -0400 Received: from relay.sw.ru ([185.231.240.75]:54300) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1iDQyb-0006ve-Ed; Thu, 26 Sep 2019 06:26:21 -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 1iDQyX-0003nC-3w; Thu, 26 Sep 2019 13:26:17 +0300 From: Vladimir Sementsov-Ogievskiy To: qemu-block@nongnu.org Subject: [PATCH v14 04/14] block/backup: improve comment about image fleecing Date: Thu, 26 Sep 2019 13:26:04 +0300 Message-Id: <20190926102614.28999-5-vsementsov@virtuozzo.com> X-Mailer: git-send-email 2.21.0 In-Reply-To: <20190926102614.28999-1-vsementsov@virtuozzo.com> References: <20190926102614.28999-1-vsementsov@virtuozzo.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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: kwolf@redhat.com, vsementsov@virtuozzo.com, wencongyang2@huawei.com, xiechanglong.d@gmail.com, qemu-devel@nongnu.org, armbru@redhat.com, den@openvz.org, mreitz@redhat.com, jsnow@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.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) | -- 2.21.0