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

[Eric Blake]

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.  Visually, if everything is local, qemu normally needs only two 
block layer entries:

qcow2 format layer => file protocol layer

But you can also make qemu use four block layer entries, since the raw 
layer is a normally passthrough layer (unless you are also using it for 
it's ability to support an offset within a larger file, such as reading 
from a tar file):

raw format layer => qcow2 format layer => raw format layer => file 
protocol layer

Then when you introduce NBD into the picture, you have the choice of 
WHERE in the four-layer system. The usual choice is:

NBD server using -f qcow2, client using -f raw:
raw format => NBD client protocol => (raw bytes) => NBD server => qcow2 
format => raw format => file protocol
(simplified to
raw format => NBD client protocol => (raw bytes) => NBD server => qcow2 
format => file protocol)

But an alternative choice is:

NBD server using -f raw, client using -f qcow2:
raw format => qcow2 format => NBD client protocol => (qcow2 bytes) => 
NBD server => raw format => file protocol
(simplified to
qcow2 format => NBD client protocol => (qcow2 bytes) => NBD server => 
raw format => file protocol)

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org