[ovirt-users] Re: Libvirt ERROR cannot access backing file after importing VM from OpenStack

On Mon, May 28, 2018 at 01:27:21PM +0300, Arik Hadas wrote: > Let me demonstrate briefly the flow for OVA: > Let's say that we have a VM that is based on a template and has one disk > and one snapshot, so its volume-chain would be: > T -> S -> V > (V is the volume the VM writes to, S is the backing file of V and T is the > backing file of S). > When exporting that VM to an OVA file we want the produced tar file to be > comprised of: > (1) OVF configuration > (2) single disk volume (preferably qcow). > > So we need to collapse T, S, V into a single volume. > Sure, we can do 'qemu-img convert'. That's what we do now in oVirt 4.2: > (a) qemu-img convert produces a 'temporary' collapsed volume > (b) make a tar file of the OVf configuration and that 'temporary' volume > (c) delete the temporary volume > > But the fact that we produce that 'temporary' volume obviously slows down > the entire operation. > It would be much better if we could "open" a stream that we can read from > the 'collapsed' form of that chain and stream it directly into the > appropriate tar file entry, without extra writes to the storage device. A custom nbdkit plugin is possible here. In fact it's almost possible using the existing nbdkit-tar-plugin[1], except that it doesn't support resizing the tarball so you'd need a way to predict the size of the final qcow2 file.

The main difficulty for modifying nbdkit-tar-plugin is working out how to resize tar files. If you can do that then it's likely just a few lines of code.