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