Hi all!
I have an idea, that not storing backing filename in qcow2 image at all may be a good
thing. I'll give some reasons and want to know what do you think about it.
1. Libvirt has to manage and keep in mind backing chains anyway.
This means, that storing this information in qcow2 header is a source of bugs when we
update it in one place but failed/forget to update in another. Of course, Libvirt is not
the only user of qemu.. But we are moving to "blockdev" anyway, when management
tool should control all node-names at least. It would be strange to not control the
relations between images in the same time.
2. backing file name specified in qcow2 metadata doesn't relate to any other thing,
and nothing rely on it.
3. calculating and updating backing file name in Qemu is a headache:
- with some options specified or with filters we risk to write json filenames into
qcow2 metadata, which is almost never what user wants. Also, json may exceed the qcow2
limitation of backing_file_size to be <= 1023
- updating it in transactional way for read-only image during reopen, when another
transactional permission update is ongoing is difficult (who know, how to do it?)
(remember recent d669ed6ab02849 "block: make bdrv_drop_intermediate() less
wrong")
4. Moving qcow2 files to another directory is a problem: you should care to update backing
file names in all dependent qcow2 images.
So, what about moving libvirt (at least) to not rely on backing file name stored in qcow2
image? Backing chain then should be in xml? Is it hard or not? Finally, will it make the
code simpler, or more difficult?
Then, if the idea is good in general, what to do on Qemu part? If we want to finally get
rid of problem code (see [3.]) we should deprecate something.. Just deprecate support for
qcow2 images with backing file specified, requiring user always specify backing chain by
hand? I don't see anything that should be changed in qcow2 format itself: no reason to
add some kind of restricted bits, etc..
--
Best regards,
Vladimir