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

[Richard W.M. Jones]

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.

Rich.

[1] 
https://manpages.debian.org/testing/nbdkit-plugin-perl/nbdkit-tar-plugin.1.en.html
    https://github.com/libguestfs/nbdkit/blob/master/plugins/tar/tar.pl

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