[openstack-dev] RFC: custom metadata per VM instance at boot time (eg for kernel args)

Daniel P. Berrange berrange at redhat.com
Wed Jan 30 14:13:29 UTC 2013

I have a need to be able to provide custom kernel command line args per
VM instance at boot time. For example, if glance is storing a kernel and
initrd pair associated with the Fedora installer, it is neccessary to
also provide a command line arg to tell the installer where to download
a kickstart automation script from, or where the install tree is located.
While some of this data could be stored in glance as metadata properties
against the 'kernel' image, some it is inherantly data that changes per
VM instance spawned. Thus I need some way to provide it at 'nova boot'


Two approaches come to mind for me

 - Add an explicit 'kernel-args' parameter to the 'nova boot' command
   and corresponding REST/RPC APIs

 - Add a generic 'properties' dict parameter to the 'nova boot' command
   and corresponding REST/RPC APIs, and define a standard property
   name 'kernel-args'.

I'm figuring the latter would be preferrable since it gives us an
extensible framework for instance metadata.  NB, this is separate
from metadata that you can alrady provide to feed through to the
cloud-init services via config-drive / metadata service. It is
metadata intended for interpretation by Nova itself, where as the
existing metadata is for interpretation by the guest OS.

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