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

[Yaniv Kaul]

On Tue, May 17, 2016 at 6:01 PM, Richard W.M. Jones <rjones at redhat.com>
wrote:

> 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.
>

I expect the input images virt-builder uses (by default) to have
preallocation=metadata.

(Then again, I expected RHEL, CentOS and Fedora cloud images to be qcow2v3
- and they aren't - just plain 0.1 compat level).
Y.


>
> 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
> virt-df lists disk usage of guests without needing to install any
> software inside the virtual machine.  Supports Linux and Windows.
> http://people.redhat.com/~rjones/virt-df/
>
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