[libvirt-users] libvirt support for qcow2 rebase?

[Eric Blake]

On 04/17/2013 07:35 AM, Skardal, Harald wrote:
> I have not found support in libvirt (nor virsh) for doing the equivalent
> of "qemu-img rebase ....".

Correct - offline image manipulation is something I'd love to see added,
but last time I worked on the issue, I realized that there is a LOT of
design work still needed.  With online image chain manipulation, we can
start a long-running job, and qemu keeps track of it for us.  So we can
restart libvirtd, and then ask qemu if the job is done yet, thanks to
qemu supporting a Unix socket monitor connection that we can reconnect
to at will.  But with offline image manipulation, a long-running job
means spawning a qemu-img child process, and that does not have an
ability to reconnect.  So libvirt itself would be responsible for
setting up enough management resources that the management of qemu-img
can survive a libvirtd restart.  Not as trivial as I'd like.

> 
>  
> 
> The use case: 
> 
> You have copied a qcow2 stack and the new files have different names or
> reside in a different directory. Therefore you need to change the
> backing file.
> 
>  
> 
> Is there a way  to do this? Is this a planned addition to libvirt?
> 

Yes, it's planned, but only when someone has enough time to make it
their highest priority effort in patching efforts.  In the meantime, you
have to settle with doing qemu-img calls yourself, then updating
libvirt's state to match your manual changes.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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