On 4/9/20 10:46 AM, Eric Blake wrote:
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because this might lead to outdated metadata; even though we usually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".
Another random thought: Should we add a header extension that records
the timestamps of an external data file? That way, if the timestamps of
the file have changed from what we recorded in our optional header, then
we can flag to the user that our metadata may be stale because of what
appears to be external edits. But that's not always going to save us -
timestamps on a block device don't behave the same as timestamps on a
POSIX file, and just because timestamps change (such as when copying a
file from one place to another) does not imply that contents have
changed. My personal take - unless adding such a header can definitely
add safety, it may not be worth the cost of complicating the standard -
this was more just documenting an idea I had even if we don't choose to
pursue it.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org
