[Libguestfs] [PATCH] v2v: Add --print-estimate option to print source size estimate.

[Nir Soffer]

On Wed, Aug 15, 2018 at 1:55 PM Richard W.M. Jones <rjones at redhat.com>
wrote:

> (Adding Stefan who implemented the subcommand)
>
> On Wed, Aug 15, 2018 at 12:44:44PM +0200, Kevin Wolf wrote:
> > Am 15.08.2018 um 12:26 hat Richard W.M. Jones geschrieben:
> > > On Tue, Aug 14, 2018 at 09:31:06PM +0300, Nir Soffer wrote:
> > > > On Tue, Aug 14, 2018 at 8:29 PM Richard W.M. Jones <
> rjones at redhat.com>
> > > > wrote:
> > > >
> > > > > This option prints the estimated size of the data that will be
> copied
> > > > > from the source disk.
> > > > >
> > > > > For interest, the test prints:
> > > > >
> > > > > 3747840 ../test-data/phony-guests/windows.img
> > > > > Estimate: 3710976
> > > > >
> > > >
> > > > Why not use qemu-img measure on the overlay?
> > >
> > > Yes this would be better, but oddly it doesn't work properly when
> > > the output format is set to 'raw':
> > >
> > >   /usr/bin/qemu-img 'measure' '-f' 'qcow2' '-O' 'raw' '--output=human'
> '/home/rjones/d/libguestfs/tmp/v2vovl62b7c2.qcow2'
> > >   required size: 6442450944
> > >   fully allocated size: 6442450944
> > >
> > > However it's OK if the output format is set to 'qcow2':
> > >
> > >   /usr/bin/qemu-img 'measure' '-f' 'qcow2' '-O' 'qcow2'
> '--output=human' '/home/rjones/d/libguestfs/tmp/v2vovla53d7c.qcow2'
> > >   required size: 1047986176
> > >   fully allocated size: 6443696128
> > >
> > > I guess it ignores sparseness of raw images, but I can't see a way to
> > > specify that on the command line.
> > >
> > > OTOH the qcow2 figure is probably a good enough guess for our purposes
> > > (ie. estimating how much data will be copied).
> >
> > 'qemu-img measure' calculates the resulting file size, not the number of
> > used blocks. I think this is because its main purpose is to create block
> > devices (like LVs) of the right size, so sparseness on a filesystem
> > level doesn't buy you anything.
>
> But if I run ‘qemu-img convert ... -O raw output.raw’ then output.raw
> will be a sparse file, and that's the file size I'd expect measure to
> give us for "required size" (of course "fully allocated size" would be
> the virtual size, and that is correct).
>
> It does look as if the block/raw-format.c:raw_measure function is wrong.
>
> In any case we can use the qcow2 estimate for our purposes as it will
> be near enough what we need (a rough estimate of the size of the copy).
>

Yes, using qcow2 as output format looks like a good estimate. In our tests
it is always less then 1.1 * virtual size.

Nir
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