Re: [libvirt] [Qemu-devel] live snapshot wiki updated

[Jes Sorensen <Jes Sorensen redhat com>]

On 07/20/11 15:46, Eric Blake wrote:
> On 07/20/2011 07:25 AM, Jes Sorensen wrote:
>>> I think if libvirt wants qemu to use an fd instead of a file name, it
>>> shouldn't pass a file name but an fd in the first place. Which means
>>> that the two that we need are support for an fd: protocol (patches on
>>> the list, need review), and a way for libvirt to override the backing
>>> file of an image.
>>
>> The problem is that QEMU will find backing file file names inside the
>> images which it will be unable to open. How do you suggest we get around
>> that?
> 
> We've already told you - qemu must have a way to be passed fds which are
> associated with names, and when a file refers to another backing file by
> name, then qemu falls back on its fd/name mapping to use the
> already-passed fd instead.  Which implies that someone else, either
> libvirt or a qemu-maintained libblockformat.so, needs to have a stable
> interface for parsing the backing file name out of an arbitrary qcow2
> file, and that this interface must work no matter how many other
> extensions are added to qcow2.

As I replied to you earlier, libvirt is *not* allowed to be messing with
internals of image files! Passing in backing file fds as a result of
libvirt messing around in internals of the image headers is utterly
broken and is not going to happen!

Jes