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 <p...@kamp.de> >> 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