[openstack-dev] [nova][glance] why do image properties control per-instance settings?

Blair Bethwaite blair.bethwaite at gmail.com
Tue Nov 25 10:15:37 UTC 2014

Hi all,

I've just been doing some user consultation and pondering a case for
use of the Qemu Guest Agent in order to get quiesced backups.

In doing so I found myself wondering why on earth I need to set an
image property in Glance (hw_qemu_guest_agent) to toggle such a thing
for any particular instance, it doesn't make any sense that what
should be an instance boot parameter (or possibly even runtime
dynamic) is controlled through the cloud's image registry. There is no
shortage of similar metadata properties, probably everything prefixed
"hw_" for a start. It looks like this has even come up on reviews
before, e.g.
The last comment from DanielB is:
"For setting per-instance, rather than doing something that only works
for passing kernel command line, it would be desirable to have a way
to pass in arbitrary key,value attribute pairs to the 'boot' API call,
because I can see this being useful for things beyond just the kernel
command line."

In some cases I imagine image properties could be useful to indicate
that the image has a certain *capability*, which could be used as a
step to verify it can support some requested feature (e.g., qemu-ga)
for any particular instance launch.

Is there similar work underway? Would it make sense to build such
functionality via the existing instance metadata API?


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