On 4/9/20 10:01 AM, Max Reitz wrote:
On 09.04.20 16:32, Eric Blake wrote:
On 4/9/20 9:10 AM, Max Reitz wrote:
What happens when an operation attempts to unmap things? Do we reject
all unmap operations when data-file-raw is set (thus leaving a cluster
marked as allocated at all times, if we can first guarantee that
preallocation set things up that way)?
No, unmap operations currently work. qcow2_free_any_clusters() passes
them through to the external data file.
The problem is that the unmap also zeroes the L2 entry, so if you then
write data to the raw file, it won’t be visible from the qcow2 side of
things. However, I’m not sure whether we support modifications of a raw
file when it is already “in use” by a qcow2 image, so maybe that’s fine.
We don't support concurrent modification. But if the guest is running
and unmaps things, then shuts off, then we edit the raw file offline,
then we restart the guest, the guest should see the results of those
offline edits.
Should it? The specification doesn’t say anything about that.
In fact, I think we have always said we explicitly discourage that
because this might lead to outdated metadata; even though we usually
meant “dirty bitmaps” by that.
Hmm. Kevin, I'd really like your opinion here. The point of the
raw-external-data flag is to state that "qemu MUST ensure that whatever
is done to this image while the guest is running is reflected through to
the raw file, so that after the guest stops, the raw file alone is still
viable to see what the guest saw". But as you say, there's a difference
between "the raw file will read what the guest saw" and "we can now edit
the raw file without regards to the qcow2 wrapper but later reuse of the
qcow2 wrapper won't be corrupted by those edits".
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org
