1. header_size: variable in qed, equals to cluster size in qcow2:
When will it be larger than 1 cluster in qed? So, what will happen to
that extra data on qed->qcow2 conversion.
If you have an feature that is used in the original image, but cannot be
represented in the new format, I think you should just get an error.
2. L2 table size: equals to L1 table size in qed, equals to cluster size
in qcow2:
we need to take it into account during conversion.
Right. I think we'll have to rewrite all of the metadata.
I wonder if we can manage to have a nice block driver interface for
in-place image conversions so that we don't only get a qed<->qcow2
converter, but also can implement the interface in e.g. VMDK and get
VMDK<->qcow2 and VMDK<->qed as well.
3. refcount table:
qcow2->qed:we do not keep refcount structure
qed->qcow2: initialize refcount structure
Yes, refcounts can be rebuilt after the mapping has been converted.
So, a qcow2->qed->qcow2 conversion means if earlier, qcow2 image was
using additional features{1-4}, all information related to that will be
lost.
We shouldn't lose anything but just abort if a conversion isn't
possible. The user can still use qemu-img convert for the more
complicated cases, for example for removing encryption or compression.