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Re: [Qemu-devel] storing machine data in qcow images?

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] storing machine data in qcow images?
Date: Tue, 22 May 2018 17:19:50 +0300

On Tue, May 22, 2018 at 09:35:55AM +0200, Gerd Hoffmann wrote:
>   Hi,
> > You must /sometimes/ supply the correct machine type.
> > 
> > It is quite dependent on the guest OS you have installed, and even
> > just how the guest OS is configured.  In general Linux is very
> > flexible and can adapt to a wide range of hardware, automatically
> > detecting things as needed. It is possible for a sysadmin to build
> > a Linux image in a way that would only work with I440FX, but I
> > don't think it would be common to see that.
> I think it would be pretty hard to actually build such an image.
> The more critical thing for linux guests is the storage driver which
> must be included into the initrd so the image can mount the root
> filesystem.  And the firmware, bios vs. uefi is more critical than
> pc vs. q35.

I think we can start by finding a location to embed a string in a qcow
image, add ability for qemu-img to set and get this string.  We can
discuss how it's formatted separately.

> > That said I'm not really convinced that using the qcow2 headers is
> > a good plan. We have many disk image formats in common use, qcow2
> > is just one. Even if the user provides the image in qcow2 format,
> > that doesn't mean that mgmt apps actually store the qcow2 file.
> > I tend to think we'd be better looking at what we can do in the context
> > of an existing standard like OVF rather than inventing something that
> > only works with qcow2. I think it would need to be more expressive than
> > just a single list of key,value pairs for each item.
> Embed OVF metadata in the qcow2 image?
> cheers,
>   Gerd

What would be helpful is if we could tell the user who wonders
how to run an image "hey you probably want flags X,Y and Z".
I can see how we could have an option to either stick
a bit of XML there, or just some QEMU flags.


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