Re: [Qemu-discuss] Converting qcow2 image on the fly to raw format

[Richard W.M. Jones]

On Mon, Jul 09, 2018 at 07:02:50PM +0200, Kevin Wolf wrote:
> Am 09.07.2018 um 18:52 hat Richard W.M. Jones geschrieben:
> > On Mon, Jul 09, 2018 at 07:38:05PM +0300, Nir Soffer wrote:
> > > We are discussing importing VM images to KubVirt. The goal is to be
> > > able to import an existing qcow2 disk, probably some appliance stored
> > > on http server, and and convert it to raw format for writing to storage.
> > > 
> > > This can be also useful for for oVirt for importing OVA, since we like to
> > > pack
> > > disks in qcow2 format inside OVA, but the user may like to use raw disks, 
> > > or
> > > for uploading existing disks.
> > > 
> > > Of course converting the image using qemu-img is easy, but requires
> > > downloading the image to temporary disk. We would like to avoid temporary
> > > disks, or telling users to convert the image.
> > > 
> > > Base on the discussion we had here:
> > > https://lists.ovirt.org/archives/list/address@hidden/thread/GNAVJ253FP65QUSOONES5XZGRIDX5ABC/#YMLSEGU7PN3MX5MUORGEGGAQLLSL4KKJ
> > > 
> > > I think this is impossible since qcow2 is not built for streaming. But 
> > > both
> > > Richard and Eric suggested some solutions.
> > > 
> > > The flow is:
> > > 
> > >     qcow2 image -- http --> importer -> raw file
> > > 
> > > Is it possible to implement the importer using qemu-img and qemu-nbd,
> > > or maybe nbdkit?
> > 
> > Strictly speaking streaming qcow2 to raw is not possible.  However
> > placing an overlay on top of the original remote image will allow
> > streaming to raw with only a modest amount of local storage consumed.
> > 
> > You can demonstrate this fairly easily:
> > 
> > $ qemu-img create -f qcow2 -b 'json: { "file.driver": "https", "file.url": 
> > "https://uk-mirrors.evowise.com/fedora/releases/28/Cloud/x86_64/images/Fedora-Cloud-Base-28-1.1.x86_64.qcow2";,
> >  "file.timeout": 10000 }' /var/tmp/overlay.qcow2
> > $ qemu-img convert -p -f qcow2 -O raw overlay.qcow2 fedora.img
> 
> This overlay stays empty, so it's pretty pointless and you could just
> directly point 'qemu-img convert' to https and the real image.

Right, indeed.  I was copying what virt-v2v does without thinking
about it enough.  virt-v2v needs the overlay because it actually wants
to write into it, and it does copy-on-read for the first phase (not
the final ‘qemu-img convert’).

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-builder quickly builds VMs from scratch
http://libguestfs.org/virt-builder.1.html