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Fri, 7 Aug 2020 08:13:37 +0000 (UTC) Date: Fri, 7 Aug 2020 10:13:35 +0200 From: Kevin Wolf To: Ying Fang Subject: Re: [PATCH] qcow2: flush qcow2 l2 meta for new allocated clusters Message-ID: <20200807081335.GA4812@linux.fritz.box> References: <20200805023826.184-1-fangying1@huawei.com> <20200806091326.GC17753@linux.fritz.box> <1c2447af-1265-c45c-6374-f20d482f51be@huawei.com> MIME-Version: 1.0 In-Reply-To: <1c2447af-1265-c45c-6374-f20d482f51be@huawei.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=kwolf@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=207.211.31.120; envelope-from=kwolf@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/08/07 02:21:37 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, 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_H3=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action 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: alex.chen@huawei.com, zhang.zhanghailiang@huawei.com, qemu-devel@nongnu.org, qemu-block@nongnu.org, mreitz@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Am 07.08.2020 um 09:42 hat Ying Fang geschrieben: > > > On 8/6/2020 5:13 PM, Kevin Wolf wrote: > > Am 05.08.2020 um 04:38 hat Ying Fang geschrieben: > > > From: fangying > > > > > > When qemu or qemu-nbd process uses a qcow2 image and configured with > > > 'cache = none', it will write to the qcow2 image with a cache to cache > > > L2 tables, however the process will not use L2 tables without explicitly > > > calling the flush command or closing the mirror flash into the disk. > > > Which may cause the disk data inconsistent with the written data for > > > a long time. If an abnormal process exit occurs here, the issued written > > > data will be lost. > > > > > > Therefore, in order to keep data consistency we need to flush the changes > > > to the L2 entry to the disk in time for the newly allocated cluster. > > > > > > Signed-off-by: Ying Fang > > > > If you want to have data safely written to the disk after each write > > request, you need to use cache=writethrough/directsync (in other words, > > aliases that are equivalent to setting -device ...,write-cache=off). > > Note that this will have a major impact on write performance. > > > > cache=none means bypassing the kernel page cache (O_DIRECT), but not > > flushing after each write request. > > Well, IIUC, cache=none does not guarantee data safety and we should not > expect that. Then this patch can be ignored. Indeed, cache=none is a writeback cache mode with all of the consequences. In practice, this is normally good enough because the guest OS will send flush requests when needed (e.g. because a guest application called fsync()), but if the guest doesn't do this, it may suffer data loss. This behaviour is comparable to a volatile disk cache on real hard disks and is a good default, but sometimes you need a writethrough cache mode at the cost of a performance penalty. Kevin