[Libguestfs] Can I specify qcow2 preallocation method with virt-builder?

[Richard W.M. Jones]

On Tue, May 17, 2016 at 05:53:31PM +0300, Yaniv Kaul wrote:
> On Tue, May 17, 2016 at 5:48 PM, Richard W.M. Jones <rjones at redhat.com>
> wrote:
> 
> > On Tue, May 17, 2016 at 03:41:53PM +0100, Richard W.M. Jones wrote:
> > > On Tue, May 17, 2016 at 05:15:57PM +0300, Yaniv Kaul wrote:
> > > > When creating qcow2 images using virt-builder, is there a way to
> > specify
> > > > the qcow2 preallocation, as possible with qemu-img create -o
> > > > preallocation=metada , for example?
> > >
> > > No .. but .. it does default to preallocation=metadata provided that
> > > the output format is qcow2:
> > >
> > >
> > https://github.com/libguestfs/libguestfs/blob/master/builder/builder.ml#L582-L585
> >
> > I should say this only applies when the image needs to be resized
> > (ie. you supply a --size parameter which != the size of the template),
> > AND if the resize is the final step that the planner comes up with.
> >
> >
> > https://rwmj.wordpress.com/2013/12/14/writing-a-planner-to-solve-a-tricky-programming-optimization-problem/
> 
> 
> Oh. I don't think there[1] is any need to resize in the code, it's just
> directly using the templates virt-builder uses, AFAIK.

The problem is:  Suppose the input template is already qcow2, and the
output format requested is qcow2, virt-builder will just copy the
input to the output and not do any preallocation.

To force a particular preallocation mode for the output image, an
extra step might be required, and that would have to be modelled in
the planner.

The blog post above is quite interesting.  Though Wordpress in its
infinite wisdom decided to break the diagram.  You can see the diagram
here: http://oirase.annexia.org/rwmj.wp.com/strips.svg

Rich.

> 
> [1]
> https://github.com/lago-project/lago-images/blob/inital-setup/bin/build.py
> 
> 
> >
> > In the general case implementing this is more complicated.  I think it
> > would require another planner transition.
> >
> > Rich.
> >
> > --
> > Richard Jones, Virtualization Group, Red Hat
> > http://people.redhat.com/~rjones
> > Read my programming and virtualization blog: http://rwmj.wordpress.com
> > virt-top is 'top' for virtual machines.  Tiny program with many
> > powerful monitoring features, net stats, disk stats, logging, etc.
> > http://people.redhat.com/~rjones/virt-top
> >

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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