On 09/10/2010 10:18 AM, Kevin Wolf wrote:
Am 10.09.2010 17:02, schrieb Anthony Liguori:
What makes us future proof is having a good feature support. qcow2
doesn't have this. We have a good way at making purely informational
changes and also making changes that break the format. Those features
are independent so they can be backported in a compatible way too.
I might have agreed that it's useful to be able to backport them
independently if we had had lots of such features added in the past. But
we haven't.
I think part of why we haven't had them is that the mechanisms aren't
very flexible.
A good example of where feature support would be very nice is for
changing the way snapshots metadata is recorded in qcow2.
It would be nice to be able to represent snapshots with a uuid. If you
added new metadata that had uuid based snapshots that were hierarchical
and added a feature bit, it would have some nice properties.
Since most images don't have snapshots, the common case would be a qcow2
that was fully backwards compatible. You would also get a graceful
failure for using a new image with an old QEMU.
You could argue that you can do the same by bumping the version number
but that really only works when all changes are additive. If you had
two features and you only backported one, badness ensues.
On it's own, I don't think feature support is enough to justify a new
image format. However, it's a nice thing to have.
Regards,
Anthony Liguori
The qcow2 mechanism for compatible changes is header extensions (used
exactly once, for the backing file format) and for incompatible changes
increasing the version number (never used so far, if you consider qcow1
and qcow2 completely independent formats, which I think they are).
Kevin
