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

[Eric Blake]

On 05/28/2018 05:27 AM, Arik Hadas wrote:

[Answering before reading the entire thread; apologies if I'm repeating 
things, or if I have to chime in again at other spots]

> 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).

I tend to write backing relationships as a left arrow, as in:

T <- S <- V

(can be read as: S depends on T, and V depends on 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.
>
> Few months ago people from the oVirt-storage team checked the qemu toolset
> and replied that this capability is not yet provided, therefore we
> implemented the workaround described above. Apparently, the desired ability
> can also be useful for the flow discussed in this thread so it worth asking
> for it again :)

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.

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