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 [1]
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. [1] https://github.com/oVirt/ovirt-engine/commit/23230a131af33bfa55a3fe828660c32f7fff04d7 On Wed, May 30, 2018 at 7:39 PM Kevin Wolf <kw...@redhat.com> wrote: > Am 30.05.2018 um 18:14 hat Arik Hadas geschrieben: > > On Wed, May 30, 2018 at 6:33 PM, Kevin Wolf <kw...@redhat.com> 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 > effort there. > > Kevin >
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