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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Fri, May 18, 2018 at 02:41:33PM -0300, Eduardo Habkost wrote:
> On Fri, May 18, 2018 at 06:09:56PM +0100, Daniel P. Berrangé wrote:
> > On Fri, May 18, 2018 at 06:30:38PM +0300, Michael S. Tsirkin wrote:
> > > Hi!
> > > Right now, QEMU supports multiple machine types within
> > > a given architecture. This was the case for many architectures
> > > (like ARM) for a while, somewhat more recently this is the case
> > > for x86 with I440FX and Q35 options.
> > > 
> > > Unfortunately this means that it's no longer possible
> > > to more or less reliably boot a VM just given a disk image,
> > > even if you select the correct QEMU binary:
> > > you must supply the correct machine type.
> > 
> > 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. Many distros build
> > and distribute disk images that can work across VMWare, KVM,
> > and VirtualBox which all have very quite different hardware.
> > Non-x86 archs may be more fussy but I don't have personal
> > experiance with them
> > 
> > Windows is probably where things get more tricky, as it is not
> > happy with disks moving between different controller types
> > for example, and you might trigger license activation again.
> 
> All I'm suggesting here is just adding extra hints that OpenStack
> can use.
> 
> I have very specific goal here: the goal is to make it less
> painful to users when OpenStack+libvirt+QEMU switch to using a
> different machine-type by default (q35), and/or when guest OSes
> stop supporting pc-i440fx.  I assume this is a goal for OpenStack
> as well.
> 
> We can make the solution to be more extensible and solve other
> problems as well, but my original goal is the one above.

Configuring the machine type is just one thing that users
would do with OpenStack though.  A simple example might be

    openstack image set \
         --property hw_disk_bus=scsi \
	 --property hw_vif_model=e1000e

Or if they're using libosinfo to set preferred devices 

    openstack image set \
         --property os_distro=fedora26

which will identify virtio-blk & virtio-net as disk+nic
respectively. Using libosinfo is more flexible than setting
the hw_disk_bus & hw_vif_model  explicitly, because libosinfo
will report multiple devices that can be used, and the virt
driver can then pick one which best suits the particular
host or hypervisor.

Setting a non-default machine type is one extra prop

    openstack image set \
         --property hw_machine_type=q35
         --property os_distro=fedora26

So while your immediate motivation is only considering the
machine type, from the Openstack POV thats only one property
out of many that users might be setting.


> > 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.
> > 
> 
> Why this OpenStack implementation detail matters?  Once the hints
> are included in the input, it's up to OpenStack to choose how to
> deal with it.

Well openstack aims to support multiple hypervisors - if there's a
choice between implementing something that is a cross-vendor standard
like OVF, or implementing something that only works with qcow2, the
latter is not very appealing to support.

> > The closest to a cross-hypervisor standard is OVF which can store
> > metadata about required hardware for a VM. I'm pretty sure it does
> > not have the concept of machine types, but maybe it has a way for
> > people to define metadata extensions. Since it is just XML at the
> > end of the day, even if there was nothing official in OVF, it would
> > be possible to just define a custom XML namespace and declare a
> > schema for that to follow.
> 
> There's nothing preventing OVF from supporting the same kind of
> hints.
> 
> I just don't think we should require people to migrate to OVF if
> all they need is to tell OpenStack what's the recommended
> machine-type for a guest image.
> 
> Requiring a different image format seems very likely to not
> fulfill the goal I stated above: it will require using different
> tools to create the guest images, and we can't force everybody
> publishing guest images to stop using qcow2.

It doesn't have to require different tools - existing tools could
create a OVF/OVA file for the disk image as part of an "export"
process.


> > > - We most likely shouldn't get backend parameters from the image
> > > 
> > > Thoughts?
> > 
> > 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.
> 
> Why you claim we are inventing something that only works with
> qcow2?

It works with a disk image format that has ability to record extra
metadata. With raw files you would have to have a separate file to
record it, likewise for any other vendor disk formats that are
not extended. 

Regards,
Daniel
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