On Thu, Feb 04, 2021 at 02:44:03PM -0600, Eric Blake wrote: > 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).
I agree, there's nothing unsafe about qemu-nbd (provided you don't use the -c option). > 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. TBH I would use nbdsh. For example to overwrite the sector at 1M in a qcow2 image with "1"s: $ qemu-img create -f qcow2 test.qcow2 10M $ nbdsh -c 'h.connect_systemd_socket_activation(["qemu-nbd","-t","-f","qcow2","test.qcow2"])' \ -c 'h.pwrite(b"1"*512, 1024*1024)' and to show it was really overwritten: $ nbdcopy -- [ qemu-nbd -f qcow2 test.qcow2 ] - | hexdump -C 00000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| * 00100000 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 |1111111111111111| * 00100200 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................| * 00a00000 Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-df lists disk usage of guests without needing to install any software inside the virtual machine. Supports Linux and Windows. http://people.redhat.com/~rjones/virt-df/