[ClusterLabs] Antw: [EXT] Re: What's a "transition", BTW?

[Reid Wahl]

On Mon, Jan 18, 2021 at 11:18 PM Ulrich Windl <
Ulrich.Windl at rz.uni-regensburg.de> wrote:

> >>> Ken Gaillot <kgaillot at redhat.com> schrieb am 18.01.2021 um 19:29 in
> Nachricht
> <1047fd943be77f4a6fd4cd4dd19b65d1550512f8.camel at redhat.com>:
> > On Fri, 2021‑01‑15 at 11:40 +0100, Ulrich Windl wrote:
> >> Hi!
> >>
> >> With a cluster recheck interval, I see periodic log messages like
> >> this:
> >> Jan 15 11:05:50 h19 pacemaker‑controld[4804]:  notice: State
> >> transition S_TRANSITION_ENGINE ‑> S_IDLE
> >> Jan 15 11:15:50 h19 pacemaker‑controld[4804]:  notice: State
> >> transition S_IDLE ‑> S_POLICY_ENGINE
> >
> > The "transition" terminology is a little confusing. Note that the above
> > uses of it are just in the normal sense, i.e. the controller state
> > changed.
> >
> > The controller uses a finite state machine to keep track of what it's
> > doing now and next. Going from "transition engine" to "idle" means it
> > finished whatever needed to be done in that transition (in the more
> > technical Pacemaker sense). Going from "idle" to "police engine" means
> > it is ready to re‑invoke the scheduler to re‑check whether anything
> > needs to be done.
> >
> >> Jan 15 11:15:50 h19 pacemaker‑schedulerd[4803]:  notice: Watchdog
> >> will be used via SBD if fencing is required and stonith‑watchdog‑
> >> timeout is nonzero
> >> Jan 15 11:15:50 h19 pacemaker‑schedulerd[4803]:  notice: Calculated
> >> transition 596, saving inputs in /var/lib/pacemaker/pengine/pe‑input‑
> >> 41.bz2
> >> Jan 15 11:15:50 h19 pacemaker‑controld[4804]:  notice: Processing
> >> graph 596 (ref=pe_calc‑dc‑1610705750‑978) derived from
> >> /var/lib/pacemaker/pengine/pe‑input‑41.bz2
> >> Jan 15 11:15:50 h19 pacemaker‑controld[4804]:  notice: Transition 596
> >> (Complete=3, Pending=0, Fired=0, Skipped=0, Incomplete=0,
> >> Source=/var/lib/pacemaker/pengine/pe‑input‑41.bz2): Complete
> >>
> >> The "transition" number increases each time, while there is visible
> >> no action to be performed. So what's in such a "transition"? Couldn't
> >> the cluster skip those lines if there's nothing to do?
> >>
> >> Regards,
> >> Ulrich
> >
> > "Transition" as Pacemaker uses it in a technical sense is what you
> > called in a different post an "action plan". A transition is all
> > actions needed to bring the cluster to the desired state (as defined by
> > the configuration), given everything known about the cluster at the
> > moment (represented by the complete CIB including configuration and
> > status).
> >
> > The controller starts a new transition whenever something interesting
> > happens (like a resource monitor failure), when a transition action
> > returns an unexpected result (like a start failing instead of
> > succeeding), and periodically (according to cluster‑recheck‑interval).
> >
> > In any case, it's possible there's nothing to do, so the transition has
> > no actions. It's still a record that the cluster checked whether
> > anything needed to be done, and decided no. I have considered lowering
> > the log message to info level in that case, though ‑‑ that probably
> > makes sense.
>
> If its something that is expected to happen frequently under normal
> conditions, I also think "info" instead of "notice" would be OK as well,
> but
> what about pe-input?
> Is a new file required even if there's nothing to do?


Nope. For example, nothing's been happening in my cluster. The transition
number increments, but the pe-input file stays the same.

# grep 'Calculated transition' /var/log/pacemaker/pacemaker.log | tail -n 5
Jan 18 22:12:13 fastvm-rhel-8-0-23 pacemaker-schedulerd[7699]
(pcmk__log_transition_summary at pcmk_sched_allocate.c:2897) notice:
Calculated transition 1003, saving inputs in
/var/lib/pacemaker/pengine/pe-input-376.bz2
Jan 18 22:27:13 fastvm-rhel-8-0-23 pacemaker-schedulerd[7699]
(pcmk__log_transition_summary at pcmk_sched_allocate.c:2897) notice:
Calculated transition 1004, saving inputs in
/var/lib/pacemaker/pengine/pe-input-376.bz2
Jan 18 22:42:13 fastvm-rhel-8-0-23 pacemaker-schedulerd[7699]
(pcmk__log_transition_summary at pcmk_sched_allocate.c:2897) notice:
Calculated transition 1005, saving inputs in
/var/lib/pacemaker/pengine/pe-input-376.bz2
Jan 18 22:57:13 fastvm-rhel-8-0-23 pacemaker-schedulerd[7699]
(pcmk__log_transition_summary at pcmk_sched_allocate.c:2897) notice:
Calculated transition 1006, saving inputs in
/var/lib/pacemaker/pengine/pe-input-376.bz2
Jan 18 23:12:13 fastvm-rhel-8-0-23 pacemaker-schedulerd[7699]
(pcmk__log_transition_summary at pcmk_sched_allocate.c:2897) notice:
Calculated transition 1007, saving inputs in
/var/lib/pacemaker/pengine/pe-input-376.bz2


> I could imagine reusing
> the last number if the last transition had no actions other than
> monitor/probe.
> Of course that would not work if inputs are interleaved (the next begins
> before the last one has finished).
>
> Regards,
> Ulrich
>
>
> > ‑‑
> > Ken Gaillot <kgaillot at redhat.com>
> >
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-- 
Regards,

Reid Wahl, RHCA
Senior Software Maintenance Engineer, Red Hat
CEE - Platform Support Delivery - ClusterHA
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