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

[Eduardo Habkost <ehabkost@redhat.com>]

On Mon, May 21, 2018 at 09:18:17PM +0100, Daniel P. Berrangé wrote:
> 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

Nice.  Are these just hypothetical examples, or something that
already works?


> 
> 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.

Agreed.


> > > 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.

I still don't understand why you claim this would only work with
qcow2.  If somebody wants to implement the same functionality in
OVF, it's also possible.


> > > 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.

Requiring a new "export" step that wasn't required before is
requiring a different tool, isn't it?


> > > > - 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. 

So this could work with both qcow2 and OVF (and maybe other
formats if others want to extend them), wouldn't it?

-- 
Eduardo