On 01/19/2017 11:25 AM, Alex Schultz wrote:
On Thu, Jan 19, 2017 at 8:27 AM, Matt Riedemann
<mrie...@linux.vnet.ibm.com> wrote:
Sylvain and I were talking about how he's going to work placement
microversion requests into his filter scheduler patch [1]. He needs to make
requests to the placement API with microversion 1.4 [2] or later for
resource provider filtering on specific resource classes like VCPU and
MEMORY_MB.
The question was what happens if microversion 1.4 isn't available in the
placement API, i.e. the nova-scheduler is running Ocata code now but the
placement service is running Newton still.
Our rolling upgrades doc [3] says:
"It is safest to start nova-conductor first and nova-api last."
But since placement is bundled with n-api that would cause issues since
n-sch now depends on the n-api code.
If you package the placement service separately from the nova-api service
then this is probably not an issue. You can still roll out n-api last and
restart it last (for control services), and just make sure that placement is
upgraded before nova-scheduler (we need to be clear about that in [3]).
But do we have any other issues if they are not packaged separately? Is it
possible to install the new code, but still only restart the placement
service before nova-api? I believe it is, but want to ask this out loud.
Forgive me as I haven't looked really in depth, but if the api and
placement api are both collocated in the same apache instance this is
not necessarily the simplest thing to achieve. While, yes it could be
achieved it will require more manual intervention of custom upgrade
scripts. To me this is not a good idea. My personal preference (now
having dealt with multiple N->O nova related acrobatics) is that these
types of requirements not be made. We've already run into these
assumptions for new installs as well specifically in this newer code.
Why can't we turn all the services on and they properly enter a wait
state until such conditions are satisfied?
Simply put, because it adds a bunch of conditional, temporary code to
the Nova codebase as a replacement for well-documented upgrade steps.
Can we do it? Yes. Is it kind of a pain in the ass? Yeah, mostly because
of the testing requirements.
But meh, I can whip up an amendment to Sylvain's patch that would add
the self-healing/fallback to legacy behaviour if this is what the
operator community insists on.
I think Matt generally has been in the "push forward" camp because we're
tired of delaying improvements to Nova because of some terror that we
may cause some deployer somewhere to restart their controller services
in a particular order in order to minimize any downtime of the control
plane.
For the distributed compute nodes, I totally understand the need to
tolerate long rolling upgrade windows. For controller nodes/services,
what we're talking about here is adding code into Nova scheduler to deal
with what in 99% of cases will be something that isn't even noticed
because the upgrade tooling will be restarting all these nodes at almost
the same time and the momentary failures that might be logged on the
scheduler (400s returned from the placement API due to using an unknown
parameter in a GET request) will only exist for a second or two as the
upgrade completes.
So, yeah, a lot of work and testing for very little real-world benefit,
which is why a number of us just want to more forward...
Best,
-jay
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