[ClusterLabs] RFC: allowing soft recovery attempts before ignore/block/etc.
abeekhof at redhat.com
Tue Oct 4 18:34:05 EDT 2016
On Wed, Oct 5, 2016 at 7:03 AM, Ken Gaillot <kgaillot at redhat.com> wrote:
> On 10/02/2016 10:02 PM, Andrew Beekhof wrote:
> >> Take a
> >> look at all of nagios' options for deciding when a failure becomes
> > I used to take a very hard line on this: if you don't want the cluster
> > to do anything about an error, don't tell us about it.
> > However I'm slowly changing my position... the reality is that many
> > people do want a heads up in advance and we have been forcing that
> > policy (when does an error become real) into the agents where one size
> > must fit all.
> > So I'm now generally in favour of having the PE handle this "somehow".
> Nagios is a useful comparison:
> check_interval - like pacemaker's monitor interval
> retry_interval - if a check returns failure, switch to this interval
> (i.e. check more frequently when trying to decide whether it's a "hard"
> max_check_attempts - if a check fails this many times in a row, it's a
> hard failure. Before this is reached, it's considered a soft failure.
> Nagios will call event handlers (comparable to pacemaker's alert agents)
> for both soft and hard failures (distinguishing the two). A service is
> also considered to have a "hard failure" if its host goes down.
> high_flap_threshold/low_flap_threshold - a service is considered to be
> flapping when its percent of state changes (ok <-> not ok) in the last
> 21 checks (= max. 20 state changes) reaches high_flap_threshold, and
> stable again once the percentage drops to low_flap_threshold. To put it
> another way, a service that passes every monitor is 0% flapping, and a
> service that fails every other monitor is 100% flapping. With these,
> even if a service never reaches max_check_attempts failures in a row, an
> alert can be sent if it's repeatedly failing and recovering.
since we're overhauling this functionality anyway, do you think we need to
add an equivalent of retry_interval too?
> >> If you clear failures after a success, you can't detect/recover a
> >> resource that is flapping.
> > Ah, but you can if the thing you're clearing only applies to other
> > failures of the same action.
> > A completed start doesn't clear a previously failed monitor.
> Nope -- a monitor can alternately succeed and fail repeatedly, and that
> indicates a problem, but wouldn't trip an "N-failures-in-a-row" system.
> >> It only makes sense to escalate from ignore -> restart -> hard, so maybe
> >> something like:
> >> op monitor ignore-fail=3 soft-fail=2 on-hard-fail=ban
> > I would favour something more concrete than 'soft' and 'hard' here.
> > Do they have a sufficiently obvious meaning outside of us developers?
> > Perhaps (with or without a "failures-" prefix) :
> > ignore-count
> > recover-count
> > escalation-policy
> I think the "soft" vs "hard" terminology is somewhat familiar to
> sysadmins -- there's at least nagios, email (SPF failures and bounces),
> and ECC RAM. But throwing "ignore" into the mix does confuse things.
> How about ... max-fail-ignore=3 max-fail-restart=2 fail-escalation=ban
I could live with that :-)
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