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Thu, 6 Aug 2020 09:13:28 +0000 (UTC) Date: Thu, 6 Aug 2020 11:13:26 +0200 From: Kevin Wolf To: Ying Fang Subject: Re: [PATCH] qcow2: flush qcow2 l2 meta for new allocated clusters Message-ID: <20200806091326.GC17753@linux.fritz.box> References: <20200805023826.184-1-fangying1@huawei.com> MIME-Version: 1.0 In-Reply-To: <20200805023826.184-1-fangying1@huawei.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 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 = 2020/08/06 03:10:56 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=ham 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 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. Kevin