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Excerpts from Day, Phil's message of 2014-01-24 04:24:11 -0800:
> > >
> > > Cool. I like this a good bit better as it avoids the reboot. Still, this is a rather
> > large amount of data to copy around if I'm only changing a single file in Nova.
> > >
> > 
> > I think in most cases transfer cost is worth it to know you're deploying what
> > you tested. Also it is pretty easy to just do this optimization but still be
> > rsyncing the contents of the image. Instead of downloading the whole thing
> > we could have a box expose the mounted image via rsync and then all of the
> > machines can just rsync changes. Also rsync has a batch mode where if you
> > know for sure the end-state of machines you can pre-calculate that rsync and
> > just ship that. Lots of optimization possible that will work fine in your just-
> > update-one-file scenario.
> > 
> > But really, how much does downtime cost? How much do 10Gb NICs and
> > switches cost?
> > 
> 
> It's not as simple as just saying "buy better hardware" (although I do have a vested interest in that approach ;-)  - on a compute node the Network and Disk bandwidth is already doing useful work for paying customers.   The more overhead you put into that for updates, the more disruptive it becomes.
> 

Agreed. The question becomes whether you should reserve a portion of
your resources for updates or let them push you into over-subscription.
Either way, those are business decisions.

And once we have a working system and we can say "this costs X bitcoins",
we can make a clear justification for somebody to spend developer time
to push X downward.