Better late than never - thank you all for the input, it was very useful!
With the ability to measure the collapsed form of a volume chain in the qcow2 format, we managed to simplify and improve the process of creating an OVA significantly. What we do now is:
1. Measure the collapsed qcow2 volumes that would be written to the OVA
2. Create the OVA file accordingly
3. Write the "metadata" of the OVA - headers and OVF, while skipping the reserved places for disks
4. Mount each reserved place for a disk as a loopback device and convert the volume-chain directly to it 
This saves us from allocating temporary disks and extra copies.
That change would hopefully land in one of the next updates of oVirt 4.2.
Am 30.05.2018 um 18:14 hat Arik Hadas geschrieben:
> On Wed, May 30, 2018 at 6:33 PM, Kevin Wolf <firstname.lastname@example.org> wrote:
> > I think the problem is that we're talking about two different things in
> > one thread. If I understand correctly, what oVirt does today is:
> > 1. qemu-img convert to create a temporary qcow2 image that merges the
> > whole backing chain in a single file
> > 2. tar to create an temporary OVA archive that contains, amongst others,
> > the temporary qcow2 image. This is a second temporary file.
> > 3. Stream this temporary OVA archive over HTTP
> Well, today we suggest users to mount a shared storage to multiple hosts
> that reside in different oVirt/RHV deployments so they could export
> VMs/templates as OVAs to that shared storage and import these OVAs from the
> shared storage to a destination deployment. This process involves only #1
> and #2.
> The technique you proposed earlier for writing disks directly into an OVA,
> assuming that the target size can be retrieved with 'qemu-img measure',
> sounds like a nice approach to accelerate this process. I think we should
> really consider doing that if that's as easy as it sounds.
Writing the image to a given offset in a file is the example that I gave
further down in the mail:
> > You added another host into the mix, which just receives the image
> > content via NBD and then re-exports it as HTTP. Does this host actually
> > exist or is it the same host where the original images are located?
> > Because if you stay local for this step, there is no need to use NBD at
> > all:
> > $ ./qemu-img measure -O qcow2 ~/images/hd.img
> > required size: 67436544
> > fully allocated size: 67436544
> > $ ./qemu-img create -f file /tmp/test.qcow2 67436544
> > Formatting '/tmp/test.qcow2', fmt=file size=67436544
> > $ ./qemu-img convert -n --target-image-opts ~/images/hd.img
> > driver=raw,file.driver=file,file.filename=/tmp/test.qcow2,offset=65536
> > hexdump verifies that this does the expected thing.
> But #3 is definitely something we are interested in because we expect the
> next step to be exporting the OVAs to a remote instance of Glance that
> serves as a shared repository for the different deployments. Being able to
> stream the collapsed form of a volume chain without writing anything to the
> storage device would be fantastic. I think that even at the expense of
> iterating the chain twice - once to map the structure of the jump tables
> (right?) and once to stream the whole data.
If the target is not a stupid web browser, but something actually
virt-related like Glance, I'm sure it can offer a more suitable protocol
than HTTP? If you could talk NBD to Glance, you'd get rid of the
streaming requirement. I think it would make more sense to invest the