On 5/22/2017 1:50 PM, Matt Riedemann wrote:
On 5/22/2017 12:54 PM, Jay Pipes wrote:
Hi Ops,
I need your feedback on a very important direction we would like to
pursue. I realize that there were Forum sessions about this topic at
the summit in Boston and that there were some decisions that were
reached.
I'd like to revisit that decision and explain why I'd like your
support for getting rid of the automatic reschedule behaviour entirely
in Nova for Pike.
== The current situation and why it sucks ==
Nova currently attempts to "reschedule" instances when any of the
following events occur:
a) the "claim resources" process that occurs on the nova-compute
worker results in the chosen compute node exceeding its own capacity
b) in between the time a compute node was chosen by the scheduler,
another process launched an instance that would violate an affinity
constraint
c) an "unknown" exception occurs during the spawn process. In
practice, this really only is seen when the Ironic baremetal node that
was chosen by the scheduler turns out to be unreliable (IPMI issues,
BMC failures, etc) and wasn't able to launch the instance. [1]
The logic for handling these reschedules makes the Nova conductor,
scheduler and compute worker code very complex. With the new cellsv2
architecture in Nova, child cells are not able to communicate with the
Nova scheduler (and thus "ask for a reschedule").
To be clear, they are able to communicate, and do, as long as you
configure them to be able to do so. The long-term goal is that you don't
have to configure them to be able to do so, so we're trying to design
and work in that mode toward that goal.
We (the Nova team) would like to get rid of the automated rescheduling
behaviour that Nova currently exposes because we could eliminate a
large amount of complexity (which leads to bugs) from the
already-complicated dance of communication that occurs between
internal Nova components.
== What we would like to do ==
With the move of the resource claim to the Nova scheduler [2], we can
entirely eliminate the a) class of Reschedule causes.
This leaves class b) and c) causes of Rescheduling.
For class b) causes, we should be able to solve this issue when the
placement service understands affinity/anti-affinity (maybe
Queens/Rocky). Until then, we propose that instead of raising a
Reschedule when an affinity constraint was last-minute violated due to
a racing scheduler decision, that we simply set the instance to an
ERROR state.
Personally, I have only ever seen anti-affinity/affinity use cases in
relation to NFV deployments, and in every NFV deployment of OpenStack
there is a VNFM or MANO solution that is responsible for the
orchestration of instances belonging to various service function
chains. I think it is reasonable to expect the MANO system to be
responsible for attempting a re-launch of an instance that was set to
ERROR due to a last-minute affinity violation.
**Operators, do you agree with the above?**
Finally, for class c) Reschedule causes, I do not believe that we
should be attempting automated rescheduling when "unknown" errors
occur. I just don't believe this is something Nova should be doing.
I recognize that large Ironic users expressed their concerns about
IPMI/BMC communication being unreliable and not wanting to have users
manually retry a baremetal instance launch. But, on this particular
point, I'm of the opinion that Nova just do one thing and do it well.
Nova isn't an orchestrator, nor is it intending to be a "just
continually try to get me to this eventual state" system like Kubernetes.
If we removed Reschedule for class c) failures entirely, large Ironic
deployers would have to train users to manually retry a failed launch
or would need to write a simple retry mechanism into whatever
client/UI that they expose to their users.
**Ironic operators, would the above decision force you to abandon Nova
as the multi-tenant BMaaS facility?**
Thanks in advance for your consideration and feedback.
Best,
-jay
[1] This really does not occur with any frequency for hypervisor virt
drivers, since the exceptions those hypervisors throw are caught by
the nova-compute worker and handled without raising a Reschedule.
Are you sure about that?
https://github.com/openstack/nova/blob/931c3f48188e57e71aa6518d5253e1a5bd9a27c0/nova/compute/manager.py#L2041-L2049
The compute manager handles anything non-specific that leaks up from the
virt driver.spawn() method and reschedules it. Think
ProcessExecutionError when vif plugging fails in the libvirt driver
because the command blew up for some reason (sudo on the host is
wrong?). I'm not saying it should, as I'm guessing most of these types
of failures are due to misconfiguration, but it is how things currently
work today.
[2]
http://specs.openstack.org/openstack/nova-specs/specs/pike/approved/placement-claims.html
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Not to sound like we don't have a united front here, but I want to
restate the concern I expressed this morning when talking about this.
I'm not an operator and don't have the background or experience there.
The 95% number thrown around at the summit was made up, as far as I
know. There is no published data that I'm aware of which says someone
tested reschedules at scale and in 95% of cases they were due to the
situation described in (a) above.
We're less than three weeks from the p-2 milestone. Feature freeze is
July 27. That is plenty of time (ideally) to get this code done and
merged. However, I don't want to underestimate the number of weird
things that are going to come out of this pretty large change in how
things work, especially when multiple cells and quotas changes are
happening.
Therefore I'm on the side of being conservative here and allowing
reschedules within a cell for now. I think long-term it'd be a good idea
to disable reschedules by default for new installs, and for people that
really need them (or feel more secure by having them) then they can turn
them on. But I'd rather see that gradually phased out once we see how
things are working for awhile (at least a release).
Yes that means possible duplication and technical debt, but I think
we've always accepted some of that, at least temporarily, for large
changes so we can ease the transition.
--
Thanks,
Matt
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