Sure applications change with versions.  However, they normally also
provide useful error messages when something that was previously valid
no longer is.  This is *not* the case here.  There is *no* error
message.

I'm not saying that users shouldn't provide valid XML when defining a
VM.  However, I am saying that if the XML being provided was previously
valid and no longer is, that some clear indication about the change in
validity should be given.  There is no indication being given by the
application.

Yes, I believe the current behavior should change.  No, I'm not
disagreeing that this is a necessary security fix, nor am I saying that
in the end this is somehow an undesired fix.  What I am disputing is how
the change is currently being handled.  I do see that an effort has been
made to migrate older configurations, and it does appear likely that
this effort will catch the majority of cases.  However, it won't catch
all cases and the current handling leaves all remaining users wondering
what the hell is going on with no guidance.

I still see two points of contention.  The first, and largest point of
contention, is that due to the behavior change, previously valid XML
configurations will now fail to operate as expected and will do so
without any indication of why they are failing to operate.  A change
such as this should generate a log entry or something that would at
least guide a user to documentation about the change and possible
resolution.  Perhaps an entry could be kicked out to the
/var/log/libvirt/qemu/ per machine log file.  Something along the lines
of:

WARNING: no drive image file format specified, defaulting to raw mode.
Please specify drive image format.  For more information please see:
....

Second, I don't believe a blurb in the release notes alone is sufficient
for this change.  Why?  Because, as indicated in the first point, there
is no output from the application to indicate that the expected behavior
has changed.  Additionally, you've indicated that the existing behavior
in lucid will be changing.  As I understand it, this will be a simple
security update.  A change in behavior like this is definitely not
something one would expect from a security update, not without a rather
"in your face" notice about it.

-- 
libvirt no longer probes chained backing stores
https://bugs.launchpad.net/bugs/656173
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