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Tue, 18 Aug 2020 08:47:13 +0000 (UTC) Date: Tue, 18 Aug 2020 10:47:12 +0200 From: Kevin Wolf To: Nir Soffer Subject: Re: [RFC PATCH 06/22] qemu-nbd: Use raw block driver for --offset Message-ID: <20200818084712.GB6865@linux.fritz.box> References: <20200813162935.210070-1-kwolf@redhat.com> <20200813162935.210070-7-kwolf@redhat.com> MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=kwolf@redhat.com X-Mimecast-Spam-Score: 0.003 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: none client-ip=205.139.110.61; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/08/18 02:16:14 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -40 X-Spam_score: -4.1 X-Spam_bar: ---- X-Spam_report: (-4.1 / 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_H2=-1, SPF_HELO_NONE=0.001, SPF_NONE=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: QEMU Developers , qemu-block , Max Reitz Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Am 17.08.2020 um 19:19 hat Nir Soffer geschrieben: > On Thu, Aug 13, 2020 at 7:36 PM Kevin Wolf wrote: > > > Instead of implementing qemu-nbd --offset in the NBD code, just put a > > raw block node with the requested offset on top of the user image and > > rely on that doing the job. > > > > This does not only simplify the nbd_export_new() interface and bring it > > closer to the set of options that the nbd-server-add QMP command offers, > > but in fact it also eliminates a potential source for bugs in the NBD > > code which previously had to add the offset manually in all relevant > > places. > > > > Just to make sure I understand this correctly - > > qemu-nbd can work with: > > $ qemu-nbd 'json:{"driver": "file", "filename": "test.raw"}' > > And: > > $ qemu-nbd 'json:{"driver": "raw", "file": {"driver": "file", > "filename": "test.raw"}}' > > I assumed that we always create the raw node? No, the first form creates only the 'file' node without a 'raw' node on top. For all practical matters, this should be the same in qemu-img or qemu-nbd. For actually running VMs, omitting the 'raw' node where it's not needed can improve performance a little. What is true is that if you use a filename without specifying the driver (i.e. you rely on format probing), you'll get a 'raw' node on top of the 'file' node. > oVirt always uses the second form to make it easier to support offset, > size, and backing. > https://github.com/oVirt/ovirt-imageio/blob/2021164d064227d7c5e03c8da087adc66e3a577e/daemon/ovirt_imageio/_internal/qemu_nbd.py#L104 > > This also seems to be the way libvirt builds the nodes using -blockdev. libvirt actually has a BZ to avoid the 'raw' node for performance when it's not needed. > Do we have a way to visualize the internal node graph used by > qemu-nbd/qemu-img? No, but as long as you explicitly specify the driver, you get exactly what you specified. For exploring what happens, you can pass the same json: filename to QEMU (maybe with -hda) and then use the monitor to inspect the state. Kevin