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

From: Gianni Costanzi
Subject: Re: [Qemu-discuss] Multiple snapshots with one guest user
Date: Wed, 25 Jan 2017 17:20:23 +0100

On Wed, Jan 25, 2017 at 5:01 PM, Alberto Garcia <address@hidden> 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".


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.


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