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

[Kevin Wolf]

Am 30.05.2018 um 15:44 hat Eric Blake geschrieben:
> On 05/29/2018 04:18 PM, Nir Soffer wrote:
> > > You CAN get a logically collapsed view of storage (that is, what the
> > > guest would see), by using an NBD export of volume V.  Reading from that
> > > volume will then pull sectors from whichever portion of the chain you
> > > need.  You can use either qemu-nbd (if no guest is writing to the
> > > chain), or within a running qemu, you can use nbd-server-start and
> > > nbd-server-add (over QMP) to get such an NBD server running.
> > 
> > 
> > NBD expose the guest data, but we want the qcow2 stream - without
> > creating a new image.
> 
> NBD can do both. You choose whether it exposes the guest data or the qcow2
> data, by whether the client or the server is interpreting qcow2 data.

But if I understand correctly, it doesn't result in the image Nir wants.
You would only export an existing qcow2 file, i.e. a single layer in the
backing chain, this way. The question was about a collapsed image, i.e.
the disk content as the guest sees it.

The problem is that qcow2 just isn't made to be streamable. Importing a
qcow2 stream without saving it into a temporary file (or a memory buffer
as large as the image file) simply isn't possible in the general case.

Exporting to a stream is possible if we're allowed to make two passes
over the source, but the existing QEMU code is useless for that because
it inherently requires seeking. I think if I had to get something like
this, I'd probably implement such an exporter as a script external to
QEMU.

Kevin