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

[Richard W.M. Jones]

On Mon, Jul 09, 2018 at 05:52:42PM +0100, Richard W.M. Jones wrote:
> 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
> 
> There are various tricks you can play with caching and copy-on-read to
> make the process more efficient (note that copy-on-read will result in
> a full local copy, but there are maybe some optimizations we could do
> in qcow2 for that).

Sorry, I meant "in qemu".

> For the other cases such as xz-compressed remote images, I'd recommend
> using nbdkit (and/or a plugin, but maybe you can do it with the
> default plugins and filters).  An actual example is not going to fit
> into this email and I'm short of time to write it, but for some ideas
> how to do this you can look at my blog and the nbdkit docs:
> 
> https://rwmj.wordpress.com/2013/06/24/xz-plugin-for-nbdkit/
> https://github.com/libguestfs/nbdkit

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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