Re: [Qemu-discuss] Multiple snapshots with one guest user

[Alberto Garcia]

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