Re: [PATCH v2] scripts/simplebench: compare write request performance

[Vladimir Sementsov-Ogievskiy]

13.07.2020 13:26, Andrey Shinkevich wrote:
> On 13.07.2020 11:43, Vladimir Sementsov-Ogievskiy wrote:
> > 12.07.2020 19:07, Andrey Shinkevich wrote:
> > > On 11.07.2020 16:05, Vladimir Sementsov-Ogievskiy wrote:
> > > > 26.06.2020 17:31, Andrey Shinkevich wrote:
> > > > > The script 'bench_write_req.py' allows comparing performances of write
> > > > > request for two qemu-img binary files.
> > > > > An example with (qemu-img binary 1) and without (qemu-img binary 2) the
> > > > > applied patch "qcow2: skip writing zero buffers to empty COW areas"
> > > > > (git commit ID: c8bb23cbdbe32f5)
> > > > > The <unaligned> case does not involve the COW optimization.
> > > > Good, this proves that c8bb23cbdbe32f5 makes sense.
> > > >
> > > > > Suggested-by: Denis V. Lunev <den@openvz.org>
> > > > > Suggested-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> > > > > Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
> > > > > ---
> > > > > v2:
> > > > >    01: Three more test cases added to the script:
> > > > >        <simple case>
> > > > >        <general case>
> > > > >        <unaligned>
> > > > >
> > > > >   scripts/simplebench/bench_write_req.py | 201 +++++++++++++++++++++++++++++++++
> > > > >   1 file changed, 201 insertions(+)
> > > > >   create mode 100755 scripts/simplebench/bench_write_req.py
> > > > >
> > > > > diff --git a/scripts/simplebench/bench_write_req.py 
> > > > > b/scripts/simplebench/bench_write_req.py
> > > > > new file mode 100755
> > > > > index 0000000..fe92d01
> > > > > --- /dev/null
> > > > > +++ b/scripts/simplebench/bench_write_req.py
> > > > > @@ -0,0 +1,201 @@
> > > >
> > > > Next, I don't understand, are you trying to fill qcow2 image by dd directly? 
> > > > This is strange. Even if you don't break metadata, you don't change it, so all 
> > > > cluster will remain empty.
> > > I have tested and it works as designed.
> >
> > But how is it designed? You just filled unallocated clusters with some data. 
> > When you read from qcow2, you'll still read zeros, because L1/L2 tables are not 
> > filled. The random data will lay untouched.
>
> Sounds reasonable. But why do QEMU-ING INFO shows the actual size increased 
> after the dd command?

Because actual file size changed :) img-info shows two sizes: virtual disk size 
(it's unchanged of course), and real size of file, which is affected by dd of 
course.



--
Best regards,
Vladimir