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Date: Wed, 7 Nov 2018 15:02:19 +0000
From: "Richard W.M. Jones" <rjones@redhat.com>
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Subject: Re: [Qemu-devel] Change in qemu 2.12 causes qemu-img convert to NBD
 to write more data
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To: Nir Soffer <nsoffer@redhat.com>
Cc: QEMU Developers <qemu-devel@nongnu.org>, Eric Blake <eblake@redhat.com>, qemu-block <qemu-block@nongnu.org>, edgar.kaziakhmedov@virtuozzo.com

On Wed, Nov 07, 2018 at 04:56:48PM +0200, Nir Soffer wrote:
> Wed, Nov 7, 2018 at 4:36 PM Richard W.M. Jones <rjones@redhat.com> wrote:
> 
> > 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.
> >
> 
> There is no way to detect the capability, ioctl(BLKZEROOUT) always
> succeeds, falling back to manual zeroing in the kernel silently
> 
> Even if we could, sending zero on the wire from qemu may be even
> slower,

Yes this is a very good point.  Sending zeroes would be terrible.

Rich.

-- 
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