On Wed, Jan 25, 2017 at 5:01 PM, Alberto Garcia <be...@igalia.com> wrote:
> On Wed, Jan 25, 2017 at 04:16:22PM +0100, Gianni Costanzi wrote:
>
> > > > what's the purpose of specifying the size when you're creating
> > > > an image with a backing file? Is it simply ignored?
> > >
> > > The (virtual) sizes of the new image and the backing image can be
> > > different. If you don't specify anything then the new image will
> > > have the same size.
>
> > I still don't understand what it means. If I have a base image of
> > 10 GB with and it is almost full of data (it has a 10GB partition
> > almost full, for example) and then I create a new image with 2 GB of
> > size based on the first one.
>
> Then you get a 2GB image. When you create an image with qemu-img you
> get the (virtual) size that you asked for.
>
> When QEMU reads from the virtual disk and a particular chunk of data
> is not in the new image then it reads it from the backing image.
>
> The practical effect is that you have a new image with the first 2GB
> of the backing image.
>
> > If I boot my os from the new Image I still see 10 GB partition,
> > right?
>
> Well, the OS will probably say that you have a 2GB disk with a 10GB
> partition, because that's what's written in the partition table, but
> as soon as you try to use it you'll get an "attempt to access beyond
> end of device".
>
> Berto
>
>
Thank you very much, this is very clear! Then I think that it can be only
useful to specify a larger size,
if you want to enlarge the FS within the new image.
Gianni
