[ClusterLabs] Informing RAs about recovery: failed resource recovery, or any start-stop cycle?

[Adam Spiers]

Andrew Beekhof <abeekhof at redhat.com> wrote:
> On Mon, Jun 13, 2016 at 9:34 PM, Adam Spiers <aspiers at suse.com> wrote:
> > Andrew Beekhof <abeekhof at redhat.com> wrote:
> >> On Wed, Jun 8, 2016 at 6:23 PM, Adam Spiers <aspiers at suse.com> wrote:
> >> > Andrew Beekhof <abeekhof at redhat.com> wrote:
> >> >> On Wed, Jun 8, 2016 at 12:11 AM, Adam Spiers <aspiers at suse.com> wrote:
> >> >> > We would also need to ensure that service-enable is called on start
> >> >> > when necessary.  Perhaps we could track the enable/disable state in a
> >> >> > local temporary file, and if the file indicates that we've previously
> >> >> > done service-disable, we know to run service-enable on start.  This
> >> >> > would avoid calling service-enable on every single start.
> >> >>
> >> >> feels like an over-optimization
> >> >> in fact, the whole thing feels like that if i'm honest.
> >> >
> >> > Huh ... You didn't seem to think that when we discussed automating
> >> > service-disable at length in Austin.
> >>
> >> I didn't feel the need to push back because RH uses the systemd agent
> >> instead so you're only hanging yourself, but more importantly because
> >> the proposed implementation to facilitate it wasn't leading RA writers
> >> down a hazardous path :-)
> >
> > I'm a bit confused by that statement, because the only proposed
> > implementation we came up with in Austin was adding this new feature
> > to Pacemaker.
> 
> _A_ new feature, not _this_ new feature.
> The one we discussed was far less prone to being abused but, as it
> turns out, also far less useful for what you were trying to do.

Was there really that much significant change since the original idea?
IIRC the only thing which really changed was the type, from "number of
retries remaining" to a boolean "there are still some retries" left.
I'm not sure why the integer approach would be far less open to abuse,
or even why it would have been far less useful.  I'm probably missing
something.

[snipped]

> >> >> why are we trying to optimise the projected performance impact
> >> >
> >> > It's not really "projected"; we know exactly what the impact is.  And
> >> > it's not really a performance impact either.  If nova-compute (or a
> >> > dependency) is malfunctioning on a compute node, there will be a
> >> > window (bounded by nova.conf's rpc_response_timeout value, IIUC) in
> >> > which nova-scheduler could still schedule VMs onto that compute node,
> >> > and then of course they'll fail to boot.
> >>
> >> Right, but that window exists regardless of whether the node is or is
> >> not ever coming back.
> >
> > Sure, but the window's a *lot* bigger if we don't do service-disable.
> > Although perhaps your question "why are we trying to optimise the
> > projected performance impact" was actually "why are we trying to avoid
> > extra calls to service-disable" rather than "why do we want to call
> > service-disable" as I initially assumed.  Is that right?
> 
> Exactly.  I assumed it was to limit the noise we'd be generating in doing so.

Sort of - not just the noise, but the extra delay introduced by
calling service-disable, restarting nova-compute, and then calling
service-enable again when it succeeds.

[snipped]

> >> > The masakari folks have a lot of operational experience in this space,
> >> > and they found that this was enough of a problem to justify calling
> >> > nova service-disable whenever the failure is detected.
> >>
> >> If you really want it whenever the failure is detected, call it from
> >> the monitor operation that finds it broken.
> >
> > Hmm, that appears to violate what I assume would be a fundamental
> > design principle of Pacemaker: that the "monitor" action never changes
> > the system's state (assuming there are no Heisenberg-like side effects
> > of monitoring, of course).
> 
> That has traditionally been the considered a good idea, in the vast
> majority of cases I still think it is a good idea, but its also a
> guideline that has been broken because there is no other way for the
> agent to work *cough* rabbit *cough*.
> 
> In this specific case, I think it could be forgivable because you're
> not strictly altering the service but something that sits in front of
> it.  start/stop/monitor would all continue to do TheRightThing(tm).
> 
> > I guess you could argue that in this case,
> > the nova server's internal state could be considered outside the
> > system which Pacemaker is managing.
> 
> Right.

Well, if you're OK with bending the rules like this then that's good
enough for me to say we should at least try it :)

But how would you avoid repeated consecutive invocations of "nova
service-disable" when the monitor action fails, and ditto for "nova
service-enable" when it succeeds?  Earlier in this thread I proposed
the idea of a tiny temporary file in /run which tracks the last known
state and optimizes away the consecutive invocations, but IIRC you
were against that.

> >> I'm arguing that trying to do it only failure is an over optimization
> >> and probably a mistake.
> >
> > OK.
> >
> > [snipped]
> >
> >> >> > The new feature will be obscure enough that
> >> >> > noone would be able to use it without reading the corresponding
> >> >> > documentation first anyway.
> >> >>
> >> >> I like your optimism.
> >>
> >> As a general rule, people do not read documentation.
> >> They see an option, decide what it does based on the name and move on
> >> if some limited testing appears to confirm their theory.
> >
> > How would else they see the option without reading the documentation?
> 
> Same way I've discovered things in the past in similar environments:
> env | sort -u

They could also read the source.  Possible?  Sure.  Likely to happen
often?  Not so much.

> > It seems pretty unlikely that someone outside the OpenStack HA
> > community would happen to read the NovaCompute RA source code, notice
> > it, and decide to use it without doing any further research.
> 
> Ah, but people are already advocating that it should default to "=no"
> everywhere else.

Sorry, I don't follow what you mean by this.

> These things always snowball.
> 
> >  And if
> > they did, and somehow got it wrong, that would still be caught when
> > they submitted the usage to the resource-agents project.
> 
> Not all agents come back to us.

Which is why I said:

> >  And if they
> > didn't submit the RA for review anywhere then the chances are high
> > that they have other serious problems with it ...

[snipped]

> >> > So what are you suggesting?  That we should deliberately avoid making
> >> > any progress, based on nebulous fear of other people making stupid
> >> > mistakes in ways that we can't even think of?
> >>
> >> Its not nebulous, we've seen for many years now how these things get
> >> used and how hard it is to walk back design errors.  You don't need to
> >> maintain it or deal with the people 5 or 10 years from now that are
> >> still getting themselves into trouble as a result, so of course you
> >> set the bar lower.
> >>
> >> Heck, I've seen how long it took people to stop abusing "target_rc" in
> >> their monitor functions (hint, it wasn't voluntary -
> >> https://github.com/beekhof/pacemaker/commit/46a65a8 ).
> >
> > OK then, I'll defer to your experience in this area, even though it
> > seems a bit overly paranoid to me.
> 
> 7 years of "no, you need to return the true state of the service, not
> what target_rc contains" is overly paranoid?

Return codes are a fundamental part of every RA action, so it's
unavoidable that every RA author has to be aware of them and use
them.  That's totally different to this case of some obscure optional
environment variable.  That's my opinion anyway, but you don't have to
agree :-)

> >> >  I'm totally open to
> >> > other ideas, but I'm not hearing any yet.
> >>
> >> I'm saying that this is not progress.
> >> That even the usecase the feature was designed for shouldn't use it.
> >> That the service should be unconditionally disabled on stop and
> >> enabled on start.
> >> That the desire for conditional enabling/disabling is rooted in the
> >> unnecessary recovery optimization of an unrecoverable system.
> >>
> >> Clear enough?
> >
> > That's a lot clearer, thanks.  We discussed this topic at the HA
> > meeting today and agreed to try this approach.  Hopefully you will be
> > proven correct and there will be no need for further optimization :-)
> >
> > BTW I disagree with the characterization of *all* these scenarios as
> > "unrecoverable" (even though it's correct for some of them), but
> > that's a minor nitpick which probably isn't worth discussing.
> 
> I'd class them as unrecoverable because whatever we've already tried
> (restarting a set of services, including nova compute, several times)
> clearly isn't achieving anything.

OK, so by "unnecessary recovery optimization of an unrecoverable
system" you meant "unnecessary recovery optimization of a system which
we already tried and failed to recover", whereas I thought you meant
"unnecessary recovery optimization of a failed system which we haven't
tried to recover yet".  But the optimization we are talking about
(i.e. skipping "nova service-disable") would only happen during
recovery attempts, before we finally give up, so it only applies to
the latter meaning, not the former.

IOW, in the case where a restart actually succeeds, we had a
*recoverable* system.

> Whatever the problem is, it is outside of the cluster's ability to
> compensate for.

In the non-recoverable case, I agree.