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

[Eric Blake]

On 11/7/18 6:13 AM, Richard W.M. Jones wrote:
> (I'm not going to claim this is a bug, but it causes a large, easily
> measurable performance regression in virt-v2v).

I haven't closely looked at at this email thread yet, but a quick first 
impression:


> In qemu 2.12 this behaviour changed:
>
>    $ nbdkit --filter=log 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
>        193 Zero
>       1246 Write
>
> It now zeroes the whole disk up front and then writes data over the
> top of the zeroed blocks.
>
> The reason for the performance regression is that in the first case we
> write 6G in total.  In the second case we write 6G of zeroes up front,
> followed by the amount of data in the disk image (in this case the
> test disk image contains 1G of non-sparse data, so we write about 7G
> in total).

There was talk on the NBD list a while ago about the idea of letting the 
server advertise to the client when the image is known to start in an 
all-zero state, so that the client doesn't have to waste time writing 
zeroes (or relying on repeated NBD_CMD_BLOCK_STATUS calls to learn the 
same).  This may be justification for reviving that topic.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org