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As part of the work to object-ify the image metadata dicts, I'm looking
at the current way the libvirt driver fetches image metadata for an
instance, in cases where the compute manager hasn't already passed it
into the virt driver API. I see 2 methods that libvirt uses to get the
image metadata

 - nova.utils.get_image_from_system_metadata(instance.system_metadata)

     It takes the system metadata stored against the instance
     and turns it into image  metadata.


- nova.compute.utils.get_image_metadata(context,
                                         image_api,
                                         instance.image_ref,
					 instance)

     This tries to get metadata from the image api and turns
     this into system metadata

     It then gets system metadata from the instance and merges
     it from the data from the image

     It then calls nova.utils.get_image_from_system_metadata()

     IIUC, any changes against the image will override what
     is stored against the instance



IIUC, when an instance is booted, the image metadata should be
saved against the instance. So I'm wondering why we need to have
code in compute.utils that merges back in the image metadata each
time ?

Is this intentional so that we pull in latest changes from the
image, to override what's previously saved on the instance ? If
so, then it seems that we should have been consistent in using
the compute_utils get_image_metadata() API everywhere.

It seems wrong though to pull in the latest metadata from the
image. The libvirt driver makes various decisions at boot time
about how to configure the guest based on the metadata. When we
later do changes to that guest (snapshot, hotplug, etc, etc)
we *must* use exactly the same image metadata we had at boot
time, otherwise decisions we make will be inconsistent with how
the guest is currently configured.

eg if you set  hw_disk_bus=virtio at boot time, and then later
change the image to use hw_disk_bus=scsi, and then try to hotplug
a new drive on the guest, we *must* operate wrt hw_disk_bus=virtio
because the guest will not have any scsi bus present.

This says to me we should /never/ use the compute_utils
get_image_metadata() API once the guest is running, and so we
should convert libvirt to use nova.utils.get_image_from_system_metadata()
exclusively.

It also makes me wonder how nova/compute/manager.py is obtaining image
meta in cases where it passes it into the API and whether that needs
changing at all.


Regards,
Daniel
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