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



On Tue, Jul 19, 2011 at 3:30 PM, Jes Sorensen <Jes Sorensen redhat com> wrote:
> On 07/19/11 16:24, Eric Blake wrote:
>> [adding the libvir-list]
>> On 07/19/2011 08:09 AM, Jes Sorensen wrote:
>>> Urgh, libvirt parsing image files is really unfortunate, it really
>>> doesn't give me warm fuzzy feelings :( libvirt really should not know
>>> about internals of image formats.
>>
>> But even if you add new features to qemu to avoid needing this in the
>> future, it doesn't change the past - libvirt will always have to know
>> how to parse image files understood by older qemu, and so as long as
>> libvirt already knows how to do that parsing, we might as well take
>> advantage of it.
>
> What has been done here in the past is plain wrong. Continuing to do it
> isn't the right thing to do here.
>
>> Besides, I feel that having a well-documented file format, so that
>> independent applications can both parse the same file with the same
>> semantics by obeying the file format specification, is a good design goal.
>
> We all know that documentation is rarely uptodate, new features may not
> get added and libvirt will never be able to keep up. The driver for a
> file format belongs in QEMU and nowhere else.

It should be a goal to avoid dependencies in multiple layers of the
stack because it becomes are to add new features - they require
coordinated changes in multiple layers.  Having both QEMU and libvirt
know the internals of image files is such a multi-dependency.  If I
want to add a new format or change an existing format I have to touch
both layers.

For fd-passing perhaps we have an opportunity to use a callback
mechanism (QEMU request: filename -> libvirt response: fd) and do all
the image format parsing in QEMU.

Stefan


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