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Subject: Re: [PATCH] qemu-img: add seek and -n option to dd command Message-ID: <3dff97ae-5172-d9ca-509f-2a520c2841b5@redhat.com> Date: Thu, 4 Feb 2021 14:44:03 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0 MIME-Version: 1.0 In-Reply-To: <567ab8bb-b452-d6c2-dec7-bc2cf1e34ec8@kamp.de> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=eblake@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=63.128.21.124; envelope-from=eblake@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -32 X-Spam_score: -3.3 X-Spam_bar: --- X-Spam_report: (-3.3 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.351, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.182, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, 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: kwolf@redhat.com, "Richard W.M. Jones" , qemu-devel@nongnu.org, mreitz@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 2/4/21 2:09 PM, Peter Lieven wrote: > Am 02.02.21 um 16:51 schrieb Eric Blake: >> On 1/28/21 8:07 AM, Peter Lieven wrote: >>> Signed-off-by: Peter Lieven >> Your commit message says 'what', but not 'why'. Generally, the one-line >> 'what' works well as the subject line, but you want the commit body to >> give an argument why your patch should be applied, rather than blank. >> >> Here's the last time we tried to improve qemu-img dd: >> https://lists.gnu.org/archive/html/qemu-devel/2018-08/msg02618.html > > > I was not aware of that story. My use case is that I want to be > > able to "patch" an image that Qemu is able to handle by overwriting > > certain sectors. And I especially do not want to "mount" that image > > via qemu-nbd because I might not trust it. I totally want to avoid that the host > > system tries to analyse that image in terms of scanning the bootsector, partprobe, > > lvm etc. pp. qemu-nbd does not have to mount an image (yes, one use of qemu-nbd is to use -c /dev/nbdX to get the kernel to mount it; but other uses are to expose the NBD image in user-space only with no kernel involvement, and therefore no system mount efforts). Another thing you might try is libnbd, which now includes a utility nbdcopy. It should make it easier to overwrite a portion of an NBD image using only user-space actions. I'm not sure if Rich has got it doing partial file overwrites yet (.../me goes and compiles the latest git checkout... nope, still a TODO item to implement subsetting), but it may be possible to combine nbdkit's --filter=offset with the full NBD image in order to then easily point nbdcopy to only the subset you care about. Definitely some ideas worthy of implementation. > >> >> where I also proposed adding seek=, and fixing skip= with count=. Your >> patch does not do the latter. But the bigger complaint back then was >> that 'qemu-img copy' should be able to do everything, and that qemu-img >> dd should then just be a thin shim around 'qemu-img copy', rather than >> having two parallel projects that diverge in their implementations. > > > understood. I was not aware of an issue with skip and count. > > The patch works for me and I wanted to share it. But when I read > > the thread it seems that it would be a difficult task to get it merged. Just because we're reluctant to improve qemu-img dd without also improving qemu-img copy does not mean that your improvements are unwanted. And hacking on nbdcopy may be faster than waiting for qemu-img to catch up to where we want it to go. > > >> >> Your patch does not have the typical '---' divider and diffstat between >> the commit message and the patch proper; this may be a factor of which >> git packages you have installed, but having the diffstat present makes >> it easier to see at a glance what your patch touches without reading the >> entire email. I had to go hunting to learn if you added iotest coverage >> of this new feature... >> >> ...and the answer was no, you didn't. You'll need to add that in v2 >> (see the link to my earlier attempt at modifying dd for an example). > > > I did not. Maybe I accidently killed the '---' divider. If I will make a V2 I will add > > an I/O test. > > > Thanks for your suggestions, Good luck! > > Peter > > > -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org