Re: Antw: Re: Q: dampening explained?

On Wed, Sep 10, 2014 at 03:00:15PM +1000, Andrew Beekhof wrote:
> >>>>> What are the precise semantics of "dampening" (attrd_updater -d)?

> > Basic idea of "attr_updater -d delay" is in fact
> > "wait for the dust to settle",
> > especially for attribute based constraints,
> > as used with the ping RA.
> > 
> > Current behaviour (afai understand it):
> > 
> > If a new update comes in while a previous update is still "pending",
> > the timer of the new update is *ignored*,
> > the new value is queued instead, and
> > the original timer is restarted.
> 
> I think thats what I wrote but longer :)

Absolutely :)

> > (UNLESS the "new" value is the same as the currently pending value.
> > Otherwise values would never reach the cib if the update interval
> > is shorter than the dampening interval.)
> > 
> > Value is put into the cib only if the original timer actually expires,
> > or attrd "flushes out" all current values for some other reason.
> > 
> > -----
> > 
> > Problem:
> > for "flapping" or otherwise continually changing values,
> > new values won't make it into the cib for "a long time".
> 
> Do we have many attributes like that?

I've seen it once for ping attributes with a dieing switch.
I guess that qualifies as "Not around here.".

> > Workaround:
> > use a dampening interval shorter than the update interval.
> > 
> >  Problem with that workaround: you may still hit the same undesired
> >  situations you could reach with immediately updating the values, you
> >  did not wait for "the dust to settle".
> > 
> > 
> > Proposal:
> > 
> > add a "update deadline" along with the dampening, which would normally
> > be sufficiently larger, and count from the original update (this timer
> > would not be restarted).
> > 
> > Optionally feed all updates into an aggregate function.
> > Default aggregate function may be "latest value".
> > 
> > Once the "update deadline" is reached,
> > (submitted values still changing; flapping? ...)
> > write out the current "aggregate" value anyways.


-- 
: Lars Ellenberg
: LINBIT | Your Way to High Availability
: DRBD/HA support and consulting http://www.linbit.com

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