On Wed, Mar 23, 2016 at 09:20:06AM +0100, Dominik Csapak wrote: > I have a strange issue with qemu-img convert which corrupts my images, and > found nothing relevant online. > Maybe someone here can help me.
Please post your QEMU command-line (ps aux | grep qemu while the guest is running). Please also post your host distro and kernel version (uname -r). > My problem: > > I have a Windows 10 VM with a hd image file of type RAW with cache mode > writeback. > I boot it up, write a bunch of data (a few gigabytes with crystal disk mark) > and shutdown. > As soon as the qemu/kvm process is gone i do a > "qemu-img convert -p -f raw -O raw vmdisk.raw vmdisk2.raw" Have you tried replacing qemu-img convert with cp? > When it finishes i do a compare: > "cmp vmdisk.raw vmidsk2.raw" > The output is: > "vmdisk.raw vmdisk2.raw differ: byte yyyyy, line zzzzz" > this it tells me that the files are different. > this manifests by the fact that sometimes in windows on the new disk i get > filesystem errors or damaged files. > (depending on where the writes were) > > now the strange part: > it does not happen with linux guests > it does not happen when i do a "echo 3 > /proc/sys/vm/drop_caches" before > the qemu-img command > it does not happen when i have the "-t none" or "-t writethrough" flags on > my qemu-img command > it DOES happen when i do a sync before the qemu-img command > the "-T" parameter makes no difference > it does also not happen when i cp the file > also when i wait a few minutes before the qemu-img command it works > correctly > > now the real questions: > is writeback with windows not recommended (i would assume after a guest > shutdown it would be safe) > what do the -t and -T flags on qemu-img convert and why does it bypass the > host page cache (apparently?) > why does it work when i specify no caching on the target > (this baffles me, since it would seem that the source cache is the culprit, > not the target) > > i apologize for the long message > and hope someone can explain this to me > > with kind regards > Dominik Csapak > >
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