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

[Markus Armbruster <armbru@redhat.com>]

Eduardo Habkost <ehabkost@redhat.com> writes:

> On Mon, May 21, 2018 at 07:44:40PM +0100, Daniel P. Berrangé wrote:
>> On Mon, May 21, 2018 at 03:29:28PM -0300, Eduardo Habkost wrote:
>> > On Sat, May 19, 2018 at 08:05:06AM +0200, Markus Armbruster wrote:
>> > > Eduardo Habkost <ehabkost@redhat.com> writes:
>> > > 
>> > > [...]
>> > > > About being more expressive than just a single list of key,value
>> > > > pairs, I don't see any evidence of that being necessary for the
>> > > > problems we're trying to address.
>> > > 
>> > > Short history of a configuration format you might have encountered:
>> 
>> [snip]
>> 
>> > > How confident are we a single list of (key, value) is really all we're
>> > > going to need?
>> > > 
>> > > Even if we think it is, would it be possible to provide for a future
>> > > extension to trees at next to no cost?
>> > 
>> > I'm confident that a list of key,values is all we need for the
>> > current problem.
>> 
>> I'm not convinced. A disk image may work with Q35 or i440fx,  or
>> work with any of virtio, ide or sata disk. So that already means
>> values have to be arrays, not scalars. You could do that with a
>> simple key,value list, but only by defining a mapping of arrays
>> into a flattened form. eg do we allow repeated keys, or do we
>> allow array indexes on keys. 
>
> No problem, we can support trees if it's necessary.
>
>
>> > The point here is to allow users to simply copy an existing disk
>> > image, and it will contain enough hints for a cloud stack to
>> > choose reasonable defaults for machine-type and disk type
>> > automatically.  Requiring the user to perform a separate step to
>> > encapsulate the disk image in another file format defeats the
>> > whole purpose of the proposal.
>> 
>> It doesn't have to mean more work for the user - the application
>> that is used to create the image can do that on their behalf.
>> oVirt for example can import/export OVA files, containing OVF
>> metadata. I could imagine virt-manager, and other tools adding
>> export ability without much trouble if this was deemed a desirable
>> thing. Bundling gives ability to have multiple disk images in one
>> archive, which is something OVF does.
>
> I have the impression that "the application that is used to
> create the image" is a very large set.  It can be virt-manager,
> virt-install, virt-manager, or even QEMU itself.
>
> Today people can simply create a VM on virt-manager, or run QEMU
> manually, and upload the qcow2 image directly from its original
> location (they don't need to copy/export it).  Don't we want the
> same procedure to keep working instead of requiring users to use
> another tool?

Today, I can take the disk out of my old computer, put it into my new
computer, and it just works.  Don't we want the same procedure to keep
working forever?

Sadly, wanting something badly enough doesn't make actual solutions any
easier :)

My point is: disk images (real or virtual) keep working in different
hardware contexts by a mixture of flexibility built into system software
on the image, disciplined evolution of hardware (real or virtual), and
dumb luck.  It works until it doesn't.  And then you get to tinker.

With OVF, you solve the problem further up the stack: you do virtual
appliances instead of disk images.

How much space that leaves for useful solutions at the level of QEMU I
can't say.