Re: [Qemu-devel] Change in qemu 2.12 causes qemu-img convert to NBD to write more data

["Richard W.M. Jones" <rjones@redhat.com>]

Another thing I tried was to change the NBD server (nbdkit) so that it
doesn't advertise zero support to the client:

  $ nbdkit --filter=log --filter=nozero memory size=6G logfile=/tmp/log \
      --run './qemu-img convert ./fedora-28.img -n $nbd'
  $ grep '\.\.\.$' /tmp/log | sed 's/.*\([A-Z][a-z]*\).*/\1/' | uniq -c 
   2154 Write

Not surprisingly no zero commands are issued.  The size of the write
commands is very uneven -- it appears to be send one command per block
of zeroes or data.

Nir: If we could get information from imageio about whether zeroing is
implemented efficiently or not by the backend, we could change
virt-v2v / nbdkit to advertise this back to qemu.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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