[openstack-dev] [nova] ability to set metadata on instances (but config drive is not updated)

Joshua Harlow harlowja at fastmail.com
Tue Jun 21 22:12:09 UTC 2016

Agreed, it appears supported right-now (whether intentional or not),

So the question at that point is what can we do to make it better...

I think we all agree that the config-drive probably shouldn't have the 
equivalent of the metadata service in it; because if the metadata 
service can change over time, and the config-drive can't then it's a bad 
equivalent and that should be rectified, either by making it an 
equivalent or making it be accepted that it is not, and ideally trimming 
the data in it to not confuse people (ie by providing static networking 
configuration only, and leaving the items that can be dynamic to the 
dynamic metadata service).

Of course this whole mess IMHO gets into, 'why don't we just have an 
agreed up agent that can be used'; because that's what the metadata 
service is starting to become (a crappy version of an agent); but I digress.

Chris Friesen wrote:
> On 06/19/2016 11:57 AM, Clint Byrum wrote:
>> Excerpts from Joshua Harlow's message of 2016-06-17 15:04:38 -0700:
>>> Now if I am using the configdrive instead of the metadata server at that
>>> special/magic ip that same metadata never seems to change (I assume the
>>> configdrive would have to be 'ejected' and then a new configdrive
>>> created and then that configdrive 'reinserted'); was anyone aware of a
>>> bug that would solve this (it does appear to be a feature difference
>>> that could/should? be solved)?
>> That's not a bug. AFAIK, it was never intended to be updated.
> <snip>
>>> Why this is something useful (from my view) is that we (at godaddy) have
>>> a cron job that polls that metadata periodically and it generates a
>>> bunch of polling traffic (especially when each VM does this) and that
>>> traffic could be removed if such a 'eject' and 'reinsert' happens
>>> instead (since then the cron job could become a small program that
>>> listens for devices being inserted/removed and does the needed actions
>>> then, which is better than polling endlessly for data that hasn't
>>> changed).
>> I think you're abusing Nova to do what your config management system
>> should be doing. The config drive and metadata service are not meant
>> to be continuously polled or updated, they're for bootstrapping into
>> a continuous config management system.
> While this is true, it seems to be a commonly-requested item. I just had
> a customer rep asking whether we support updating instance metadata for
> a running instance, since apparently it's possible with AWS.
> Chris
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